What's the best unit to put in the Serpent watchtower?

Thank, that looks good.
I'd have like to use Musketeer in sniper mode, but it doesn't look like they can use battle gear while in tower.

Well, after finishing the Serpent campaign, I have to disagree. I used cannoneer on one level but starting next level onwards I used musketeer. These guys felt much better. Maybe I didn't give cannoneer enough of a chance, but musketeers have the ability to never miss and seemed better equipped to fighting units (while cannoneeres were better against buildings).

Also, put a few musketeers in sniper mode behind the towers and you have an almost impenetrable defense (and sniper musketeers have the additional advantage that they'll not run after enemies like some fools). These guys are amazing. And useful in offense too.

There are no "best" units. The game is liquid as its creator. Cannoneer deals blunt damage, where as Musketeer deals explosive damage, therefore Cannoneer would be a good choice against units, weak to blunt damage, and musketeers for the units, weak to explosive.

Cannoneers are put on tower because they have pass-through cannonballs that damage enemies in a row + they have a distance upgrade, which makes Cannoneer the most common pick. In multiplayer tower- cannoneers may attack enemy warchtowers and they do that with great power, which is their another "pro".

I haven't played that much with Serpent, but I most commonly use musketeer in towers, and commonly like to put the towers in line. There isn't much strategical plan behind this. I do this for faster killing single units, also the only way to stop Zymeth when he comes in group.

By the way, as a side note, one of my complaints about the game is that this information is missing from the game, and the manual. There is no way I'd have learnt about damage types if it wasn't for the Battle Realms wiki page. I mean a good part of the strategy is knowing what units are good against what, yet this information is completely missing if you don't look for it on the internet. Unless the original game did come with an additional reference card that's not available on GOG.

I agree, it should've been stated somewhere. I learnt it all by playing online, and it's been a much more enjoyable experience. The only "reference" card is in game cursor that changes its colors, depending on how effective your attack will be:

Gold sword/arrow: Effective attack (the unit is weak, so the attack damage will be multiplied, i.e. 150%)
Silver sword/arrow: Normal attack (the unit has no weaknesses/resistances, so the attack will deal normal damage, 100% only)
Bronze sword/arrow: Not effective attack (the unit is resistant to your attack, so the damage will be divided, i.e. 50%)

To be honest, I only understood that after 3 years of online, when I have already learnt every possible weakness and resistance. It was a funny discovery. The cursor is still unreliable though. It might show bronze when the attack will be normal, probably because it counts 25% of weakness/resistance as major (while its not much).

That's very useful to know. Thanks!