This game is a TD, a.k.a. Tower Defense, which means that a whole lot of enemies (i.e. peasants – evil, evil peasants!) move along a fixed path, and you have to stop them from getting to the end of that path (the ominous top-right corner of the screen) using whatever means possible. The method I recommend is building sea monsters along the coast, as that’s all the game lets you do. Poison towers slow enemies, ultimate towers live up to their name. Happy peasant hunting!
Python 2.6 + pyGame: download.