I've noticed the following bug, and the attached image explains it best. you can't surround a tower completely, but if you have some guns or blocks on a diagonal, it will allow the tanks to damage the tower through that corner, but you can't build a defense that allows for that corner access.
in the attached image, you can see i left a path to the tower, but the tanks never use the final 4 squares of the path, because they detonate in that corner between the last two blocks. however, if i were to remove that last block and try to put a gun in that 4-square space, it would tell me it's an invalid location to place a gun. i have to leave the path, but they don't have to use it!
no fair (i'm losing bad enough as it is!)
thanks for an addicting game - many hours spent not working!