That game has a blatant obvious design flaw though, grid based map means diagonal attacks are 2.5 times more efficient than frontal attacks.
Since you can not build diagonal walls there will always be 1 unit shooting at 1 unit in a diagonal method, and thus whittle you down at the "sides" no matter what you do.
Just saying, if you make a TD game, you better not base it on a box grid and have units coming at you from an 45° angle. is the first thing i noticed in a medium map to be honest, build fort - enemies come, 1 always survives at the diagonal and attacks you with no recourse till the wall is dead.