The first few levels start out very basic, but it does get fairly difficult. As you kill enemies, they drop money that you need to pick of to build and upgrade towers. The towers are all fairly South Park themed, ranging from "normal" things like baseball shooters to tied up cats that "cheese" the advancing foes.
The story advances after each completed level. It is told through stylized anime like sequences, with full voice acting from the few original actors that do most the voices. Something terrible has happened in South Park, and now waves and waves of all sorts of people and creatures are stopping at nothing to get through!
As you progress through the story mode, you go to many South Park inspired locations. The school, sixth grade hangout, junkyard, even the top of a mountain for an epic confrontation with the half man, half bear, half pig itself.
You will also confront hippies, crab people, homeless, gingers, aliens, and many other things from the show. Every enemy has its own weakness, so there is some strategy to your turret placement. You can also build snow mounds to build towers only. Enemies can cross snow mounds, so you can maze your enemies for maximum efficiency.
The games single player has offered my hours of entertainment, but the co-op is probably more fun. It plays the same but seems to be a bit easier because you have two people collecting coins. When not characters are not in use, they simply throw snowballs at whatever is nearby. You can almost use the characters you aren't using as mobile towers.
Almost ever character from the fourth grade class is playable after you beat the game. Each character has their own tailored special move that relates to their character. For example, Kenny specials makes everyone drop extra money, Token pulls out a bass guitar and damages enemies. Even Pip makes an appearance and nobody likes him.
Overall, it's a great game for $10. I've replayed it a few times, and as of right now, it is the only game I have every achievement on. Besides being the best South Park game available, it is definitely the best tower defense game on the Xbox Live Arcade. I highly recommend everybody checks it out to prepare for the Stick of Truth!