More work on towers:
I made towers destructible. Creeps no longer pass through them, they hit them in the same way that they hit walls.
I created an Effects sequence for the towers that includes: creating the effect, registering it to the tower's neighbors, and activating the effect when creeps enter the square. The current effect is just changing the color of the creeps, but it is enough to work. A large fraction of the the continuing work will be in adding and improving effects to create different towers.
I modified the creeps so that they power up when passing though a source. Right now, they just increase their radius, but they will increase their power, speed, etc., upon power up. I may also institute special effects, like projectiles and immunities.