I am just throwing ideas. No idea if this a good one or not. But to be more sensitive towards upgrading Support units, it will be interesting to: Make Medieval Walls immune against Battering Ram Make Renaissance Walls immune against Battering Ram and Siege Tower It is possible: the Urban defenses allow to make those two units already ineffective, so why not make this more early? Therefore, Urban defenses will be "just" some instant-max-level walls. It maybe needs Battering Ram to be able to upgrade into Siege Tower and not directly into Medic. One way to make Siege Tower more powerful is also to damage the garrison unit.