I think walls that block LOS/movement would help a lot. Right now my biggest beef with the towers is that they're too accessible. Also, this would help defenders gain an advantage and make the bases under 200 a bit tougher than they are right now. Granted 250s would have to be tweaked some, but walls would make for more interesting gameplay IMO. Of course they should have HP and damage resistance like towers, but they shouldn't be able to attack back.