I recently stumbled across the perfect tower defence layout. Admittedly, it only works on maps where the land is flat, like that of Burdigala.
I placed a straight line of a wall, not one that ends in a '<' or '>' point and placed towers so that they touched against the wall. I then placed a second length of wall so that it not only touched the towers but, in effect, sandwiched them between the walls.
Like that, the towers supply archers, who man both sets of ramparts.