With the last South Park game I played being the average Nintendo 64 game, I wasn't expecting too much from a South Park themed XBLA game. To be honest, this is also my first 'tower defence' game too. Well unless you count playing my flatmates at Age of Empires and coating the map with canon towers.
Each level consists of a couple of screens with entrances and exits. Your goal is to make sure nobody survives the journey between the two. Before the hordes come, you are given a few seconds to set up your defences and prepare. To defend your town from Dwarfs, Hippies and Old People you can set up a variety of defence towers. Each being effective against differing enemies, and each having their pros and cons. During the onslaught, you are able to play as the South Park crew and run around the levels throwing snowballs, collecting coins and improving your defences. Every character has a unique ability which you will need to familiarise yourself with, which range from whirlwind attacks to powering up other characters for a limited amount of time.
Like most good games, this one can be played with friends. Locally, or online you can play with four players. I recommend playing this with people you know; playing with strangers results in everyone cancelling each others' towers and building what they want instead, as resources are shared. With friends, you can discuss and work together to maximise your defences. Oddly enough, online you can't choose your character. The host decides.
Single player is an entertaining play through, but won't get many replays. Sure you can try for achievements and perfect scores, but once you have seen whats on offer, there isn't much incentive for a second visit.
Graphically the game is very simplistic, which is faithful to the South Park licence. Being an official game does lend to the experience a bit, mostly because of character voice overs, but doesn't change the gameplay in any way. There are some bonuses for South Park fans, such as (very) short clips from episodes, but nothing new really.
All up this is a competent game, but more of an entertaining distraction than something you will invest much time into. Really I would recommend this as a good distraction for when you have your mates over playing more serious games, not as a main course in any way.