But the tower should keep its supportive role in the game, so it should heal the garrisoned units at least. Here are already some ideas:
For the elves:
Tree outpost can't shoot anymore, and will give the garrisoned unit increased range
For the undead:
Option 1, sorcerer tower, There's some kind of mist or aura around the tower that will confuse enemies and will change flying objects.
Example: when enemy with range (trebuchet, archer) attacks this building, it get a 20-40% chance to shoot the projectile (the arrow, or rock) back to the attacker.
When enemy is close range: get a 20-40% chance to convert it for 1 turn(unit is hallucinating and will attack it's own allies). When it's converted the undead can't delete the unit to prevent abuse.
Option 2: the undead can summon ghost units in the tower (every 4-5 turns?). These units can be ghoul, lich, undead knight etc. For the undead it will looks like a ghost. But for the enemy it will look like a normal unit. The ghost can't attack and when the ghost unit is hit it will instantly die though.
The ghosts can be used as diversion or as defence, because the enemy will attack some of the ghosts instead of the real units.
For the orcs
?
What do you think of them? and If there are other ideas, please post them below