TowerMadness Zero is the free, ad-supported (and full) version of TowerMadness, a tower-defense game in which you defend a small herd of cute sheep against an advancing army of aliens (who want to turn your sheep into "a new scarf for their emperor").
The game and its interface will be familiar to fans of tower-defense games, with enemies from multiple UFOs attacking across an open plain at first, but then mixing in fixed-path and open-path challenges as the game progresses through 15 maps. You touch the map to build and upgrade towers, and you can use two fingers to zoom in and out--with the ability to zoom all the way in for a nearly first-person, on-the-ground view. Tower selection is limited on some levels, but you can ultimately choose from nine different weapons (the usual suspects, with lasers, slow effects, power boosters, anti-aircraft guns, etc.) with five potential upgrades each (and as with many similar games, the towers go offline briefly during an upgrade). As with other open-path tower-defense games, the trick is often figuring out where to array your towers to maximize the amount of time that enemies spend marching--and, hence, taking damage from the maximum number of towers. You face 16 different enemy types, on the ground and flying, with varying vulnerabilities, defenses, and powers, and the enemy AI and difficulty ramp up in a satisfying progression.
A year ago, TowerMadness would have been the best tower-defense game in the App Store, but in an ever-growing genre, this title doesn't necessarily stand out. The sharing and leaderboard options are nice (especially the ability to watch other player's replays)--as is the extensive, stat-packed help--but the 3D graphics feel more like a gimmick rather than adding to gameplay in a substantive way. It's a good game, but you can find TD games that are better-looking, better-playing, or that have a more involving story. Of course, the best thing about TowerMadness is that it's free--and for a cute, fun, challenging tower-defense game, that is an accomplishment. With more maps and additional content on the way, it could get even better.