challenge : after wave 20 (21,22,etc) can only place thorn stick thingies and use buffs. after wave 26 (27,28,etc) can not place / replace towers, and can not use buffs.
A sniper turret, light (minimum health) & speedy enemy, and a strong & fast enemy, a splash damage turret, a bomb building that has a really long cooldown, and also enemies lose kill value every loop.