So, I'm trying to understand how to tell which units go first during a battle in HoMM2.

I know that it has to do with the speed rating of the units in question and which side is the attacker but.... that's all I really know.

Can someone fill in the blanks or point me to somewhere that will? My google-fu is failing me on this one....

Thanks!

Attacker units go first.
Ultra Fast units go first (Phoenix, Champion). First go 1st ultra fast unit in Attacker's army then 1'st ultra fast unit in Defender's army, then 2nd uf unit in attacker's army and so on.
After Ultra Fast, very fast units enter battle, then fast and so on to very slow.

Does that order repeat itself as the battle goes on or does the defender get to go first in round 2?

It seems that vertical position on the battle map has some effect - what's going on there?

There are some spells that can affect units speed (Slow, Mass Slow, Haste, Mass Haste) and thus can affect the order. The order is preserved otherwise in 2nd round, 3rd rounds etc (IIRC).

I don't have the game installed so I cannot verify any of my claims, but if my memory is correct:

Ultra Fast > Very Fast > Fast > Average > Slow > Very Slow

During the first ever move, if two opposing units are tied in speed, the attacker's unit will move first. After that, for every other ties involving opposing creatures, the game will check which player made the previous move:
If the attacker just moved, it will be the defender's turn,
If the defender just moved, it will be the attacker's turn.

If two or more creatures are tied in speed in the same army, the unit that was located on the highest row when the battle began will move first.

A unit cannot move twice per round due to tempering with its speed through spells (a fast unit that has already moved this round, and was then slowed, will not move a second time when it will be the time for the slow units to make their move.)

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Attacker:
A - Average
B - Fast
C - Slow
D - Very Fast
E - Average

Defender:
1 - Fast
2 - Very Fast
3 - Fast
4 - Average
5 - Very Fast

Move order will proceed like this:
D > 2 > 5 > B > 1 > 3 > A > 4 > E > C > 2 > D > 5 > B > 1 > 3 > A > 4 > E > C...

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Those are the general rules regarding the moving order of units for Heroes of Might and Magic II (in the third game things get a little bit more complicated with the "Wait" option).

There are more rules, regarding summoned elementals, hasted, resurrected and cloned stacks which I am not familiar enough with to inform you about. Same goes with units that get cured from slowed, paralyzed, blinded or petrified status both by enemy attacks or the dispel spell.

Wow, thanks guys!

I am having a blast with this game. I also bought HoMM3 but I decided to play this one a while first.