Place turrets on the path, but also be aware of your Commander (mech) who gives special abilities.
(Credit: Screenshot by Jason Parker/CNET) 
The tower defense genre started out with Desktop Tower Defense, a Flash game you could play in your Web browser, but once the iTunes App Store opened, developers quickly realized this type of game was a perfect fit for iOS devices. Soon, tower defense games that are now iOS classics emerged, including GeoDefense, Fieldrunners, and the hugely popular Plants vs. Zombies. The touch-screen interface made iOS devices a natural platform for tower defense gaming, allowing you to place units easily with only a few taps of your finger, and the result was the perfect time-waster requiring both quick thinking and a solid strategy.
Those early games are still fun even now, but the modern entries in the Tower Defense genre add even more to the action with 3D graphics, new types of gameplay, and new ways to take advantage of today's more powerful iOS devices. Whether you're a serious tower defense aficionado or new to the genre, you'll like the direction developers have taken with the tower defense games that are now available.
This week's collection of iOS apps are all tower defense games. The first is the latest sequel to a popular franchise in which you defend against an alien invasion of the homeworld. The second offers up crisp 3D graphics as you defend a flock of sheep from an onslaught of enemies. The last is a new game that takes a different angle, challenging you to become the invader against well-defended maps, using units and abilities that keep your assault force alive.
Sentinel 3: Homeworld ($2.99 - Universal) is the latest installment (released late in 2010) of the Sentinel tower defense franchise, offering more units to choose from, new maps, and new abilities that help you stave off wave after wave of alien enemies. The interface is like many in the fixed-path tower defense genre, letting you drag and drop units on the edges of a path to fortify your defenses against each wave of enemies. Sentinel 3 comes with a 20-level, 14-map campaign and an endless mode, and also offers in-app purchases if you want to add even more levels and maps.
Sentinel 3: Homeworld starts you off slow; you only get one type of unit plus a Commander that offers bonuses to nearby units. As the game progresses you'll be able to unlock more than 20 turrets, orbital ship weapons, and automated drones. The unique Commander unit can be upgraded as well, and you'll need his higher stats and special abilities to defeat some of the higher-level assaults. Once you've unlocked a few units, you can plan for the coming invasion by setting up loadouts of units that will work best against specific types of enemies.
Overall, with new turret types, the addition of the Commander, four skill levels, OpenFeint support, and original music from Specimen A, Sentinel 3: Homeworld makes for an excellent addition to the franchise. Anyone who likes tower defense games will enjoy this well-polished title.
TowerMadness ($2.99 iPhone - $7.99 iPad) has been around for a while now, but the unique challenge and several updates over time make it a must-have for tower defense fans. Featuring 3D graphics that look great on the Retina Display, TowerMadness challenges you to defend a flock of sheep from an onslaught of attacking aliens. You can view the action from above or use a reverse-pinch gesture to zoom in on the action. You'll play on 49 included maps, but you can purchase 28 expansion maps from within the game. A recent update added 5 more maps, and two new weapon upgrades to add to the replay value.
TowerMadness offers both campaign and endless game modes along with an easy tutorial mode in the beginning to get you comfortable with the gameplay. TowerMadness is a bit different from the other games in this collection in that it is an open field rather than a fixed-path tower defense game. This means you'll need to place your units in such a way that the enemies will need to navigate around them to reach your flock of sheep. You'll quickly learn that the best strategy is to create a sort of maze with your turrets, forcing enemies to pass by a mix of turrets several times. TowerMadness also includes replays so you can analyze your strategies to see which work the best.
Tower Madness has Game Center support with 29 achievements you can earn. You can also compare your scores with other players' online, adding additional challenge to an already challenging game.
While we continue to enjoy this title, we think the pricing may be a bit steep for a game that's been around as long as it has. The game seems to go through cycles of going on sale and recently was free for a time, so maybe you'll want to wait for the next sale. Still, if you like strategy games and want an incredibly deep tower defense experience, TowerMadness offers plenty to keep you interested.
As your units move through the war-torn streets, you'll need to use repair and other abilities.
(Credit: Screenshot by Jason Parker/CNET) Fortunately, as you progress through the game, you'll gain abilities that will repair your vehicles and make it harder for enemies to hit you. You'll be able to deploy special weapons that are strong against specific enemy types, making it crucial to select your unit loadout wisely. You'll also have to select your path; Anomaly Warzone Earth has a tactical screen you'll visit before each quest to design a path through several city blocks of strategically placed turrets. Often the quest will require that you take out specific enemy types, so you'll need to design your path to maximize damage against them. To keep you well fortified, a drop ship occasionally drops health and other abilities that you'll need to tap to add to your arsenal.
Overall, Anomaly Warzone Earth is a very well-made and challenging game that turns traditional tower defense on its head. With excellent graphics and a unique take on the game genre, this title will keep you coming back to try new strategies against increasingly difficult enemies.
