While not precisely a tower defense game, Zombie at the Gates does involve a tower and wave after wave of nasty attackers. The cartoon-colored undead hordes here aren't trying to destroy the tower, though. They're coming for you: a weapon-wielding king who just wants to escape. Your salvation lies in gathering resources (wood, wool, stone) and turning your castle into a floating escape pod. Upgrade weapons and such as quickly as possible, because you're going to need them. Don't forget your zombie-stunning stomp move--it's going to save your life many times over. Godspeed, little king.