For those who aren't familiar with tower defense, the concept is simple: wave of enemies (often refered to as creeps) spawn an move towards a goal. You have to stop them by building fixed towers which will attack the enemies. There are a lot of variants in various different versions of the game but that's the common basic concept. I'd suggest playing some flash versions if you need a better idea of Tower Defense.
The first thing to do when thinking about the game is ignore that it's South Park. LGTDP is a fully features Tower Defense game. The early levels enemies move on a set path which you can build towers. The big twist here is your avatar in the game, one of the four boys from south park can throw snowballs and act effectively as a mobile tower. As they throw snowballs they charade up a special move, for example Cartman damages all enemies on screen, Kenny makes all enemies drop coins, Kyle makes all characters move faster and do more damage for a brief period and Stan restores some damage to your town. You unlock new characters as you play, they have different special attacks and different stats for how fast they move, range and damage of snowballs. After the first few levels the format changes a bit with enemies forced through areas of snow on which you can build. This lets you build mazes slow their progress. Rather than building mazes purely out of towers you have a cheap wall option so you can quickly build some complicated mazes and later build towers on top of some of your walls. In addition there are various environmental hazards you can trigger or which go off periodically (like a sewage pipe that periodically floods a canal killing all enemies caught in its wake).
Figuring out how to make best use of your building space with relatively cheap walls allows for some fairly complex strategies. It's that rare thing, an enjoyable piece of trial and error game play because it's rewarding when you come up with a good route to run creeps. Unfortunately there is also some bad trial and error gameplay, certain enemies are weak against certain towers and you have to make use of these weaknesses to succeed with your allotted resources. You don't really know what creeps are coming when beyond the next wave which often means a few tries to get your choice of towers right.
Going back to what makes it South Park is entirely set dressing. The characters are South Park characters. Style of humor is clearly South Park. You unlock short clips as you play however these are very short and often stop frustratingly short of the punchline. It's strung together by a very silly but appropriate plot.
Something fairly clear is an aggressive minimisation of down time. Towers build instantly, and you can upgrade them once at a cost. Towers are pricey compared to the amount of money you get, for example even on the survival challenge map I only had about a dozen towers when I was done. You are actually fairly dependent on the damage your characters can do by throwing snow balls. This is mixed blessing, the reduction of down time makes it a much punchier game but playing it single player can be frustrating as you are left to try and manage all four characters at once. trying to build up towers, keep your characters some where useful to attack but out of harms way, collecting drop coins and triggering environmental hazards.
You really need several sets of hands but even multilayer has its pit falls. Your resources and space is so limited that you really need a shared vision of how you are going to lay out your walls and towers. One person can just about lay out all the walls you need before the first wave spawns but it's a push so it's often easier to leave it up to one person to lay out everything than to come up with a plan cooperatively or even for them to explain what they want done. It really needs a level of communication which is more than online play can really give. So ideally you need all the players in the same room.
While I appreciate the novel twist on TD I have to say I actually prefer the more sedate pace of Pixel Junk Monsters to the frenetic pace of LGTDP both for multi player and quick fix tower defense.
I have a few other minor gripes. The overall amount of content is very limited. What is fixed barrier and open ground often isn't obvious so sometimes the creeps just walk around your clever mazes or dig through them since you've inadvertently blocked off the exit entirely. The exit isn't marked and sometimes it's not entirely obvious what counts. The bonus character and challenge levels weren't really publicised and it's frustrating that for now at least you can't even just pay some points for the extra content on what is a fairly content poor game.
LGTDP is fully featured implementation of Tower Defense with some nice ideas which deliver a punchy, if some times overly frenetic, experience. The game play is surprisingly deep but you are going to need some friend and extra pads to hand to get the most out of this game.