At this point, we're assuming that the casual fan is ready for a bit more of a hands on approach, and the South Park tower defense game provides that in spades. Early levels include prebuilt mazes, like the one pictured above, but soon you and your co-op partner (and up to two others, which is fairly rare in such games) are tasked with building your own mazes. Maximizing efficiency by combining a good maze with the right blend of towers is a very cooperative experience. Be warned, though; the same iireverent humor that makes the TV show so popular is certainly present in the game. Not recommended for the younger set, but a nice step up for an older tower defense fan, particularly one familiar with the show.
This PSN gem has the appearance of a fairly standard tower defense game, but there's much more to it than that. Certainly, there are still towers, mazes, upgrades, and the like, but there are some key differences that add both complexity and, to the more hardcore fan, a sense of fulfillment. The biggest difference is the ability to actually fight back against the invading opponents by building factories that produce units for you. These various units can be used to attack your foes wherever needed, and this twist to the gameplay turns the tower defense genre on its ear. The only real drawbacks to Comet Crash are the rather bland graphics, which can often make things difficult to make sense of in the heat of battle. Still, it's certainly worth checking out.
Once your new casual co-op tower defender has made it this far, they aren't really a casual gamer anymore. Sanctum is one of the more challenging games on this list, due to the heavy reliance on first-person shooter skills, but it is, at its core, a stellar example of the genre. Though there are relatively few maps, there are a host of different options for creating your maze, choosing the right towers, and upgrading your own suite of weapons for maximum effectiveness. Those who enjoy the analytical nature of tower defense games, particularly in the area of spending resources, will find Sanctum a triumph. If you make it this far, congratulations; you are well and truly a "hardcore gamer" now, whatever that is.
Though this list is a fairly representative selection, there are dozens of other games out there that fit in at various places on the spectrum. Xbox Live Indie fans should consider Murky Horizons, a twin stick shooter/tower defense hybrid that I've covered before on Co-Op Casual Friday, as well as Sol Survivor. Recent XBLA release Trenched is another game that will interest tower defense fans. Coming soon, we can look forward to Toy Soldiers: Cold War and Defenders of Ardania. It's a good time to be a gamer, and especially one who craves the thrill of placing towers in the best places in order to wipe out an invading army of bad guys. So good out there, gamers, and find other, casual tower defense players. Cultivate them with games like these, and you may bring a new co-op fan into the fold!