A tower that looks like a chess rook. Charges for a time, then spawns a random crowd of units, the size of which is dictated by the tower’s morale. Level the tower to increase its morale and therefore, the size of the crowd that the tower can spawn. Start at maybe 8 morale, go up to 20 or so. All units except monsters can spawn and they take on the boosts the alliance has active.
Boosts could increase health/morale of the tower, buff spawned units, make the tower do some damage on its own, heal passing troops that are allied with the tower, etc.