The "tower defense" genre arguably began with some scenarios in Warcraft, but has since become the first truly new genre we can point to as emerging from the indie games scene. Pioneered in Master of Defense, and widely available in free flash form in games like Desktop Tower Defense, a number of good commercial versions have appeared as well.
Games of this style seem to fall into two categories; ones, like Master of Defense, in which levels have different geographies and the main strategy is figuring out optimal placement for your towers; and ones like Desktop Tower Defense where the geography is open, and the main strategy is in building towers to channel the attackers (called "creeps") along a circuitous path, giving you plenty of opportunity to shoot at and kill them. Attack of the Creeps falls into the second category.
Given the number of decent free tower defense games, the question immediately arises whether a commercial version is needed; Attack of the Creeps shows why the answer is at least a qualified yes. Flash is good for what it is, but there's something to be said for a game that takes over your screen, has appealing sound, and provides smoother UI than you can get with Flash. Additionally, AoTC has more levels of tower upgrades, and a greater variety of towers (and variety means, ultimately, more complicated strategies for you to explore). Ten bucks seems a reasonable price to get a more polished and deeper game than the Flash versions, at least if you like this game style.