Garrisoned units in keeps

Which units do you garrison in keeps?
Does the unit type affect the attack power or range of the keep?
If not, then I guess it’s best to garrison the cheapest units, rather than handcannoners or crossbowmen.

It’s used almost always for villagers or other non-combat units. The unit type does not change the damage or range, it is the same for all units garrisoned as those stats are calculated by the keep as a base number + X Units.

You may garrison military units if you want to avoid losing them to a bigger army or what-have-you. Other than that, it’s generally used as villager protection, and for holding a location with the emplacement upgrades