Try to defend a colony with 40+ units and just a transport leader (5 units, 3 attacks, 12 movement). The result will be almost the same as not having any leader at all (happened to me by a mistake, lol).

I'm not sure about the correct amount, but defending leader get a unit cap bonus, and that's it all. So, having about 10 unit cap for your garrison commander is generally enough for repelling any single AI attack (assumed he has at least 10-12 attacks), but if you move one of your main leaders in (with 20+ unit cap, and can provide that and bonus units), you will still feel the difference.

Militia and native helpers (if available) will be always present on top of any leader unit cap, so you can get twice of what you can ever field and make the reserves row pretty much unmanageable, but I see no other max limit aside from leader unit cap (with some defense bonus).

I also think, it is *almost* not relevant is or not units inside the town attached to any leader or to with leader; as you may see by damaged units distribution after the battle (if you have multiple leaders with attached units in town) the defending force is assembled regardless of that.

Now that almost... I really not sure but got the feeling, the order of units in your colony unit list may be relevant, and it might be changeable by attaching/detaching units to leaders (otherwise it just historical recruit order). I had partial success of preventing damaged units to show up in next defense this way, or at least tricked myself to believe I had.

It uses one leader, and selection of available units (regardless where they attached as far they inside the town).

Usually the leader with more attacks take command if there multiple. But I haven't explored all the extreme/borderline cases, probably it some trade-off between attacks and unit cap (or possibly reputation/charisma also) with strong favor to attacks.

I don't see any hard bonus from having multiple other leaders in the colony.

I feel like to add some summary here :)

Barracks
Build level 4 Fort ASAP - you can manage to get first one by around turn #30. Only ever recruit level 4 (strength 5) units, unless badly pressed by frequent early raids.

War College
I recently invariably spend my first 30k on Leader research (2k + 4k + 8k + 16k).
Gives +8 points for 40 points on regular Leader recruit. It is totally worth it, and I do it first by the simple consideration your existing units do benefit from War College "training", but your leaders don't, so you want to do this before to start mass recruiting them. It gets very costly to add more, so let it for the later.

After that, make your priorities about this order:
- Cavalry Attack
- Infantry Defense
- Artillery Attack
- Cavalry Defense
- Infantry Attack
- Artillery Defense (you may well get along by never spending anything on this)

Leaders
Your army is only that good your leader is. I use following standardized recruits, all assuming 40 points to use:

- Garrison Leader: 10 Units, 9 Attacks
First type to recruit, obviously. Each and every colony should have one of those. Hint - assuming there level 4 colony center, it faster to transfer gold than move this type of leader - especially over mountains.
Duties: defend your colonies. Counts on the defense bonus to Unit cap. Can go out in short raids if need be. May grow to a Great Leader eventually, but not most probably.
Experience: only get lots if you raided by a neighboring AI player. Little native raids may give some early on. Use points to get more attacks first, add units later.

- Attack Leader: 15 Units, 7 Attacks
This is classic, you will see it as main type of late AI player Leaders also.
Duties: bother everyone around you. Often works in pairs for larger objectives (first one hit and kill what can, retreat, second comes in the same turn and finish the job). Best base to grow your Great Leader from.
Experience: this is the serious business, expect lots of it. Increase Attacks to 9, then Units to 21 then Attacks to 12, then Units to 24 (or even 27), add more Attacks and/or Movement after that.

- Transport Leader: 10 Units, (3 Attacks), 10 Movement Bonus
These guys are crazy fast. You need two of this type for every Attack Leader on active duty.
Duties: transport troops between colonies, replenish and reinforce Attack Leaders, transport settlers, hunt enemy Explorers. Relay troops by several of those for wast distances in single turn (lot of micro-management, feels cheat-like to do, lol).
Experience: not intended to fight he can still occasionally encounter a native raid in hostile tribe territory or in defense of newly founded distant colony. If getting some, you may want to upgrade to Expedition Leader with 12 Units, 5 Attacks, spend on even more movement after that.

- Expedition Leader: 8 Units, 5 Attacks, 8 Movement Bonus
Combat-oriented fast Leader. You may want one such as early as possible, but overall somewhat unnecessary helper type that can grow in kind of boosted Transport Leader relatively easy.
Duties: Reconnaissance in force. Patrol and explore hostile native territories, hunt retreating enemy units, launch small raids (often just to take a look on enemy troops).
Experience: Upgrade to 12 Units, add some Movement, then Attacks if you collect a lot of points somehow.

- Support Leader: 25 Units, (3 Attacks)
Simply, organize as many units as possible.
Duties: Organize troops in over-crowded front-line cities, follow Attack Leaders in long-distance missions, serve as a mobile army base to receive and dispatch units from/to Transport Leaders.
Experience: this a dead end career, it not expected to receive any.

Armies
Use balanced mix of all 3 types of units. The basics would go about this:
9 Units: 3 Cavalry, 3 Infantry, 3 Artillery
12 Units: 6 Cavalry, 3 Infantry, 3 Artillery
15 Units: 6 Cavalry, 6 Infantry, 3 Artillery
18 Units: 9 Cavalry, 6 Infantry, 3 Artillery
21 Units: 9 Cavalry, 6 Infantry, 6 Artillery or 12 Cavalry, 6 Infantry, 3 Artillery
24 Units: 12 Cavalry, 6 Infantry, 6 Artillery
27 Units: 9 Cavalry, 12 Infantry, 6 Artillery (may want 15+ Attacks to use this build effectively)
30 Units: 12 Cavalry, 12 Infantry, 6 Artillery
(I see no point in having even more... anything above 24 units is kind of silly already)

Unique bonuses

Temple of war - it just plainly overpowered in your hands, I think. AI may (and will) benefit from it but not in the way you can. Another 4 Units and 2 Attacks for each and every Leader... um, you just won the conquest, I think. Nevertheless, it loads of pure fun anyway.
Interestingly, it give twice the bonus for High Native, making the armies instantly huge, but there not much to use that many attacks on, without the artillery...
P.S. I just spot two of them on a single map, do they stack? That would be totally crazy!

Ancient Ruins - along some others, this discovery can give one or more of several combat-oriented bonuses:
- the Crusader Leader you may recruit from Churches - with better stats and even more points to spend;
- 4x level 3 (strength 4) native helpers for every battle anywhere on map (even for a lone Explorer caught in trouble)
- +2 or +3 levels bonus to your native helpers. You still need to attract them the normal way (charisma helps a lot for this, but you still need friendly village anywhere in sight), but now, instead of half-useless strength 1 or 2, they are all the way to strength 5.

Tactics

It is lot of fun to be dare and creative, but if you look for a relatively simple way to win most of your battles slowly but steadily this little manual may do it - there is one general scheme that almost surely works... And it is simple enough to describe.
You also can test it in "Combat Demo" (choose 48-50 points, increase Leader to 6 to have 9 attacks). You may not win every battle there, especially if give enemy more artillery and Infantry, less cavalry and one-two less attacks.
But you should use your strategical skills to never get in battles that balanced... ;-) ...and as soon you have any noticeable advantage over your enemy, you should win with this.

I will start with 18 Unit army of 9 Cavalry, 6 Infantry, 3 Artillery. For first, assume we are attacking.

1. turn:
Move one Cavalry to every square in second row.
Move one Cavalry, one Artillery, two Infantry to every square in first row.
(3 Cavalry remains in reserve)

2. turn:
(assuming enemy filled third row with his Cavalry, what is most probably to happen)
Charge with Cavalry from first row, together with firing Artillery and second row Cavalry. Do this for every line (if have less than 9 attacks don't fire front Cavalry, but only charging Cavalry + Artillery ).
Advance all Infantry to second row.
Move one Cavalry to every square in first row.
Now you have what I will call "standard position": 2 Cavalry and 2 Infantry at second row, one Cavalry and one Artillery in first at every line.

Every turn until enemy front line breaks:
Move back forward any retreating Infantry (or out of action if it remains at last point)
Move back one Cavalry from second line (or not, if any retreated, of course)
Charge with Cavalry (the one that was there at end of last turn) from first line, firing Artillery of that line and any active units in second line. (this may need up to 15 attacks to do in all 3 lines, if you have less, choose enemy weakest flank and attack there full force every turn, spend most remaining attacks at center, fire other flank only occasionally or just few shots, but not leave it defensive only altogether, and always repair position there as needed - and sure, it better charge fresh one, not just move retreating Cavalry back in position.

When one flank of enemy looks very vulnerable:
Only attack there with charging Cavalry and Artillery (may add one Infantry if feel a need).
If previous cleared the third line, advance Infantry there.
Charge-flank with second row Cavalry together with full force attack in center line. If the center line looks weak too, may save one Cavalry and infantry there (also, no need to charge here, can fire front Cavalry directly).
If center is cleared also and you have any units active, and attacks left, repeat with advance and flanking charge along center, aiming the far flank. It is not uncommon to advance to third row along all 3 lines at once. If both flanks look weak but center strong, you may try to advance both flanks and charge-double-flank the center. Don't be too hastily to advance to third row, if you have no pressure to do it (like defending against a raid) it is risky enough to hold back a bit even if you could.
For every line that advanced to third row:
Move back row Cavalry to second row (yes, you can do that even if pulled it back already the same turn)
Now you will have: one Cavalry and one or two Infantry in third row, two Cavalry (maybe one Infantry) in second, Artillery in first (you may add second or even third Artillery, if happen to have more in reserve or even move from other line along back row, especially if truing to advance trough center in hopes to rout enemy fast (as may be in the case of defending against a raid)).

After partial advance to third row:
(assuming your units there survive, or the square is still unoccupied by the enemy)
Charge one Cavalry and fire all Artillery forward, may add front infantry also if feel like (best one of, if have both there already)
Advance Infantry to third row if not yet there.
Pull back the front Cavalry and flank-charge the next line, together with full or partial attack along it, as described above.

After full advance to third line:
Cycle your Cavalries in third-second row to charge each turn, just like previously in second-first. It would be good now if you had two (or more) Artillery per line to back your (by now surely weakened) units. Move it along the back line if you get a chance. This phase rarely last anywhere long, enemy flanks will break easily, and then you advance again, just like previously to third line, only now you have much less risk to do so, as no more enemies in front and Artillery can't fire sideways.

Final moves
I usually try and kill as many enemies I can, and avoid taking the flag unless that a goal from the start.
Use Artillery from side you got the final, fourth row, to reinforce center.
Keep cycling your Cavalries, if you have any units you want to train more than others (like having mixed level 5 and level 4 army, or having some level 3 units on the field for whatever reason) it is time to give them one-more shot in this fight, you can happily flank-charge in fourth row with units of even one-point left, there little risk of losing units at this point (unless in front of enemy Artillery, pull out anyone very low from there, replace with as fresh units as you can find)

Well, as you noticed I ignored the obvious fact you may have loses... you will have, unless not have huge advantage in War College research. So, you still will need to interpret this creatively to account for that ;)

Variants:

- Attack in force: If you attacking and can't see any or only very few Cavalry for enemy, or think you are massively more powerful (like attacking native village) and plan for fast battle, forward two Cavalries to second row first turn, then try to kill enemy Cavalry (if any) with charge + Artillery from first line, and charge forward to third row right away if that possible, or you can pull one Cavalry back and advance infantry to assemble the "standard" position.

- Defense: you can use exactly the same moves for defensive battle as well. Be aware, the lone Cavalry in second row can become easy kill for attacker, especially if it is another European player, but it often the price you are willing to pay for getting on the second row at all, and not letting him to charge your back row right away. Forwarding two Cavalry per row may add survival chance, or only increase loses, depends who you are dealing with and with what research levels (combat tends to become insanely deadly and attack-driven in late stages). Anyway, it is highly likely the second line will be yours at end of second turn, with something much like the "standard" position, what you finish to build in third turn if necessary. It don't differ an inch are you attacking or defending from this point on.

- Small defense: you may choose to not forward any Cavalry at all to second row, and let them charge your back row. This works great against natives who have tons of weak Cavalry and no artillery. Your combined force defense is likely to stand the charge with minimum injury. Then you fire Artillery and one Infantry, if that kills or routs enemy Cavalry, flank-charge with Cavalry, advance infantry, etc. Chances are, at the end of second turn most of enemy forward Cavalry be dead and you have something very like standard position.

- Small army: most of time you won't have all units mentioned. Additional Artillery is helpful at late stages, but clearly optional (you may want additional Cavalry instead). If you have less than 9 Cavalry, pull back one, and charge with another, you only have one Cavalry and two Infantry in your front line at end of turn, but that weakens your position less you may feel about (what it increases is sudden-death risk, a little bit, as more hits will be taken by one-less unit). Likewise, you can get along with only one Infantry per line (but position without any is considerably more vulnerable, Infantry are much better defenders then Cavalry, no matter the research levels you may put on).
All this is true if you get beaten, lose and/or gradually pull back one-point-left units to avoid loses.

- Even larger army: it is easy to cycle Cavalries to your reserve and replace them with fresh ones (it may sometimes be beneficial to only allow most units to take one hit, especially when defending city against multiple, massive, repeating attacks). Much harder to do with infantry. You may force it to die in front line, and just move fresh in, even this may be problematic to do, you can't cycle charge while do that.

- A lot of infantry: if you for whatever reason have huge army with a load of Infantry but relatively little numbers of Cavalry, advance some Cavalry to second row as usual, but move 4 infantry to every square in first row for your first turn. Then, at second turn, move all that Infantry to second row, pull back the Cavalry. You won't be able to fire anything but Artillery that turn. Then, charge with Cavalry next turn, and after cycle it with another as usual, only having 4 Infantry one Cavalry forward. This tend to make somewhat longer, defensive feeling battle, unless you have extreme number of attacks - but then, this may be slow, but strong attack and good way to utilize the excess of attacks you have accumulated.

Your defensive force will feature the 10 recruited units plus militia.
It seems awkward, like an exploit, but it works^^

That's why your garrison leaders should have LOTS of attacks and almost nothing else.

.

Garrison defense unit cap bonus is +6 units.

And of course your militia units and fort cannons, those are always present, and regenerate, but only once per turn.

So, the 4 additional units will show up. Even units you requit this turn will show up. Of course, only if you have less than cap+6 units already, because the new units will go to the end of the list.

I think I have described it elsewhere here already, but the unit selection in case you do have more units available than your defensive leader cap+6 depends solely on position of those units in the unit list and not depend directly are they attached to a leader or with leader.

However, a stack of units take the place on the list of the oldest unit in that stack.

The only tool you have for organizing the list is unit age.

Organization of the unit list can become a bit of black magic art if you have lots and lots of badly injured units and expect multiple attacks on you the following turn (what you normally should have prevented, but not always possible, or even desirable in edge cases).

If no unit is attached to any leader inside the city, the units are listed in their natural order: by age, order of creation. You may want to take a look at that order by un-attaching all units first when planning out your forces before critical defense, especially if you're not intimately familiar with the forces (like after loading an old save or if there was lots of frantic movements in and out of the town recently).

To fully control with units you field, you want to have at least two, and better three or more leaders in the town: a Helper, the Garrison leader, and one or more utility leader present to organize reserves. From those, only the Defender is necessary purpose created. Helper is a good role for your pathetic starting leaders or any other low level leader you have created early, as those will naturally sort on top. In a frontline town chances are the arriving transport leader is older than anything in the town and thus a good candidate for the role temporally. Reserves leader(s) may be intentionally created, or may be just anything.

So,

How to organize defense units:

1) unless Helper leader is the oldest combat unit (listed on very top) in town all by themself, you attach the oldest combat unit to Helper leader (sorting Helper leader stack on top),

2) unless Garrison leader is himself oldest or second oldest unit, you attach second oldest combat unit to him (sorting his stack immediately after Helper's stack),

3) if organizing reserves is important and you have third leader present, you attach the third oldest combat unit to Reserves leader (sorting his stack immediately after Garrison leader stack).

# Note that if one of the mentioned two oldest combat units is older than your leaders and injured, it's though luck, you will have to field that injured unit. No, there's no way around it, but we do this to know exactly what units we field, and minimize use of other injured units if at all possible.

# Also, of course, if the third oldest combat unit happens to be one of your only three cannons (or cavalry) in town, you will look for closest non-essential unit to pin the Reserves leader as high in the list as possible.

4) fill Garrison leader to capacity with good selection of units for him to use,

5) give Helper leader five more units, for 6 total units in his stack, and no more than six, those six units will be fielded together with Garrison leader units for the first defense of the day.

6) give Reserves leader units up to Garrison leader cap+6 or to capacity if he can't handle that much. Hopefully, loses and bad injuries of first battle will be replaced with selection from this list.

# I believe that units injured in the battle happened previously on the same turn are temporary demoted on the unit list, but only for some limited step. After two defensive battles in one turn the effective unit list will likely be chaotic no matter what you do.

I did some thorough experimenting by saving the game a bunch of times on a specific turn and replaying it in different ways. Even though the manual states that "any military units within the colony will assist in the colony's defense" this is wrong. There is either a bug in the game or an intentional formula that limits the # of troops that show up. Or a combination of both. Also, the above post by Enneagon isn't completely correct either.

Here's the deal.

1.) Having other leaders inside the colony, even if no units are attached to them, somehow screws up the number of units that show up to battle. I proved this doing the following. I had a leader with 10/8 (cap/attack) inside the colony and had 10 troops attached to him. I had additional troops also, and then a few other leaders hanging out in the colony. When I got attacked, only 9 troops showed up (not counting the militia, of course). You read that right: not even the full # of troops that were directly attached to the leader came out to fight. Infuriating. I then detached all the other leaders and otherwise had the exact same situation. Voila!--all 15 of my regular troops showed up to defend this time, plus the militia.

2.) Having a leader with "low" leadership capacity will lead to fewer troops showing up to defend. (But it's not necessarily+6.) I did another experiment where I had a leader with just 6 leadership, and 11 attack. (All other leaders in the colony were ejected.) You can see the screen shot attached--the Spanish leader is about to attack, I have 12 total in the colony, and 3 additional troops being built. The outcome was that only 9 troops showed up to defend. So the # that came out was capacity + 3 extra. I then replayed this turn and swapped in a leader with 10/8 cap/attack, as in 1.) above. Again, all 15 troops showed up to defend (including 3 new units that were generated from forts).

So I'm not sure what the exact formula is, but it's clear that it does not pay off to only maximize attacks for defensive leaders--later in the game, that is. Max attack points works great in the early to middle stages of the game when everyone is still using lower level troops and the militia are almost as as good as your regular troops. In the later stages, militia become cannon fodder, and it's more important that enough regular troops show up to withstand all the 5-point units of the attacker.

PS:
I think I could share the save-game files of the turn right before attack, if anyone wants to double check.

The thing is, it's crucial with leader from those you have in the colony take the command. I believe the number of attacks is the first parameter of choice. The number of units depends upon properties of that leader then. Others have no effect.

I will admit I haven't experimented with such extreme cases as 6 leadership 11 attack, but it's surprising you had only 9 units available, it should have been 12. The +6 rule have held true for me in all cases I have paid attention. I will try to experiment if opportunity occurs.

Okay, I did some testing already. And yes, learned something new about a game I play occasionally for over twenty years.

The formula of units in defense appears to be simple: leadership +50%, rounded down, capped at 29 (in presence of full garrison*?)

I found a save where my town is under attack and had a helper leader with leadership 6 in it (exactly because my apparently false belief that's the fixed bonus), it also had 40 units in it and I brought in 31 more for the total of 76 units including production from 5 forts. So I used cheat engine to give that guy a ton of experience points, saved a bunch of copies and this is what I saw:

Leadership --> Units in defense (bonus)

None --> 7
5 --> 7 (+2)
6 --> 9 (+3)
7 --> 10 (+3)
8 --> 12 (+4)
9 --> 13 (+4)
10 --> 15 (+5)
11 --> 16 (+5)
12 --> 18 (+6)
14 --> 21 (+7)
16 --> 24 (+8)
19 --> 28 (+9)
20 --> 29 (+9)
22 --> 29
30 --> 29

It's probably worth to note that Temple of War bonus is a bonus on a leader before this, apparently. (Luckily, I had a explorer pinned Temple, so I could test with and without it's effect.)

* It could be theoretically interesting further testing is the unit cap effected by the ten garrison units. Without any forts, there's what, five infantry men always present? Without a prior battle the same turn they could be lost at, that is. I also have yet to test what unit cap there is for open field battles, as largest army I have used so far was 27 units.

While at it, I ran few more experiments on defense leader selection.

My conclusion remains unchanged: number of attacks is the only criteria for choice. In case of draw, the oldest leader will take charge. Yes, that's right, the oldest, as in first recruited.

I had three leaders to play with:
(all level 4, in order of age, Leadership/Attacks/MoveBonus/Charisma/Reputation)

Willem Kieft: L6/A6/M5/C0/R0 (helper)
Dan de Rotterdam: L11/A9/M0/C0/R2 (garrison leader)
Pixie Lisa: L11/A3/M12/C0/R7 (transport)

With all three in the colony as is, I had 16 units in defense and 9 attacks. Dan had assumed command as expected.

To play with, I used cheat engine to give everyone 50 experience points. I'm reluctant to try and edit the other stats directly, as there's no saying what I'm missing by doing that. So I was limited to just increasing stats by spending experience points.

First, I increased Willem Kieft's attacks to 10 and as expected, he assumed command, giving me 9 units and 10 attacks.

Then it become interesting. I intended to make two tests at once, so I set all three at 9 attacks and increased leadership of Dan by one. If Reputation trumps unit count, Lisa will command, if unit count matters, Dan will command.

Willem Kieft: L6/A9/M5/C0/R0
Dan de Rotterdam: L12/A9/M0/C0/R2
Pixie Lisa: L11/A9/M12/C0/R7

It gave me still 9 units! It was Willem commanding again.

Just in case Charisma might be important I increased Lisa's Charisma to 10 with everything else repeated. It was still Willem commanding.

To be sure I increased Willem leadership to 20, but with only 8 attacks, so

Willem Kieft: L20/A8/M5/C0/R0
Dan de Rotterdam: L12/A9/M0/C0/R2
Pixie Lisa: L11/A9/M12/C10/R7

It was Dan's command, with 18 units and 9 attacks

Finally, just to see Lisa as commander and make double sure neither unit count or Charisma matters

Willem Kieft: L6/A8/M5/C20/R0
Dan de Rotterdam: L20/A9/M0/C0/R2
Pixie Lisa: L11/A10/M12/C0/R7

And, it was as expected, 16 units with 10 attacks, provided by Lisa.

And, no it makes not a difference who the units are attached to inside the colony. At least, not until you want to be double sure exactly with 16 units out of 40 you want to field, and that ordering is in effect regardless of who commands.