Original sprites, some enemy variety, map complexity, and a game speed adjustment button would do wonders for this. Rn there's not much to do but wait for my guys to slowly kill the same enemies again and again. Also the game crashed after like 17 or 18 waves, so didn't even get a chance to see the shop, but I cant imagine different towers will help much in this stage. Tower defense games were always a blast for me, so by no means give up on it, man.