After a Holiday delay and a few unrelated problems, I'm back at it.
I've fixed up the walls, especially, today. I found the persistent error in the AI programming that ignored the walls (it wasn't an error really an error in the program: the AI script that was being used was intended to look at the terrain, not the structures built on them; in fact, I'll have to fix the fixes later). I then made the creeps destroy the walls. The difficulties in all this were in how storing the terrain in the walls interacted with the expanding board.
Next up: base towers.
Additional work for walls:
(1) Dynamic drawings to indicate connections (right now, they just little white squares).
(2) Carrying power through the walls of different types.
(3) Indicate the power in each of the walls in the dynamic drawings -- at the right scales.
(4) Renderers need to be scale dependent for mini-map and zooming.