It's a fun game, but it does have a lot of niggling annoyances:
-You can deconstruct turrets, but not fences...very annoying, especially if you misplace a wall!
-When you click to upgrade towers, it covers a lot of the screen, meaning you can not upgrade quick enough if your character/ screen is positioned in such a way that you have to move character up/ down to upgrade turrets you couldn't select (hope that makes sense)
-rather than have a click and place method, I am sure a numbered system (ie, tapping 1 to build wall, 2 for turret and 3 for bomb) would be more than efficient and easier to manage this kind of game
-have an upgrade for the wall system? It reaches a point where the only thing you can do is rebuild walls as quick as possible and the game becomes a bit pointless after, as you can not build more turrets to fend the enemies off (and even if you did I think it would be pointless as the turrets have a certain range and building walls caps the range of turret...catch 22 :S
-bombs are useless, maybe a ranged turret could be added instead?
-no specials, like I dunno, a bomb that blows up all the enemies or a special that heals the health of your turrets/ walls
-Finally: I think you need to give the player a bit more time to rebuild the base in between levels. Unless that was intention...because the only way to survive long enough is to build enough walls to cover the turrets and we know that adding more turrets is pretty much pointless, even when upgraded. You can not expect the player to be able to build insanely fast with a defence and attack cap based on how this game runs. It would be like giving someone a machine gun and increasing the volume of zombies attacking you. Eventually, the amount of enemies you kill vs super influx of enemies are negligible. Anyways, gg, but could be better :)