Tower Defense (TD) games are one of life's guilty pleasures. The premise is simple: you start off with some form of home base and must defend that base against an endless swarm of attackers. The action is fast paced and happens in real time (these games are also called Real Time Strategies or RTS for short). With the advent of iPhone gaming, TD titles are starting to find their way into the touchscreen loving hands of casual gamers. BioDefense from Resolution Interactive is the latest TD title on the iPhone. As the title suggests, gamers must defend their base against a barrage of genetically mutated creatures. There may not be a large variety of weapons to employ to save your base from destruction, but killing mutants by the hundreds is always fun and never gets old.

When you first turn on BioDefense, you will see a quick video introducing you to the game's back-story. A comet has crash-landed on Earth, unleashing Toxiums into the world. These Toxiums have caused a few genetic mutations in the population - seven to be precise. You'll face your typical zombie as well as mutated dogs, the athletic Basher, the Fatman and even giant-land-roaming-mutant whales. Since you play off of a large overhead map, you won't really experience the differences between most of the enemies. Your attention will be too busy focused on defending your headquarters. Also, there is no way to increase magnification of the screen to really get a feel for the enemies' unique character models. However, what you will see is rivers of cartoon blood begin to emerge as you slaughter wave after wave of zombies. If you want to see the subtle differences between the Basher and the Zombie you can check the enemy database outside of the game.

The game features two modes: survival and mission. In Survival mode, gamers will face off against an endless number of monsters, hoping to last for as long as possible. You must defend your Headquarters using five different defensive tools: the Spotlight, the Toxium collector, Turret (machine gun), Artillery (rocket launcher), and Generator. BioDefense employs a visibility system, reminiscent of the Advanced Wars series. Gamers are given limited visibility and must deploy searchlights in order to see enemies or set up new equipment types. Depending on how much equipment you have deployed, you will also need to set up additional Generators for energy. Deploy too many units and your energy will quickly be drained. What the game is lacking though is a variety of weapons. When you have a game that involves outlandish creatures like mutated whales, you would expect a larger variety of creative weapons. Sadly, you will only have access to machine guns and rocket launchers to fight off mutants.

Each unit costs a number of Toxiums that you accumulate using your Toxium collector. In order to build stronger tech items, you must upgrade your tech using Toxiums. There are three levels of upgrade: the first costs 200, the second costs 400 and the highest level costs 800 Toxiums. Once you upgrade your tech level, you can then pay to upgrade each individual unit you have deployed on the playing field. This is more difficult than other TD games you may have played, which only required you to pay once to upgrade the unit. Paying a large cost to upgrade your tech just to have the ability to actually perform an upgrade seems like a needless hurdle. However, this does add to the level of difficulty, which long-time TD fans will enjoy. The endless zombie attacks are relentless and will have you glued to your screen.

The large overhead playing area is balanced out well with a map system that appears in the top right corner of the screen. The map does a good job of showing you where enemy troops are coming from as well as showing you the exact position you are currently looking at. This way you can focus your attention on trouble areas.

If survival mode isn't your style of play, there are 10 additional missions to play. These missions all have their own unique level of difficult. For starters, trying to get your tech levels up before a giant whale smashes your headquarters will take a lot of practice. The sequence of levels actually works well in training you for each successive level, so you will want to play through them in order.

BioDefense employs a level of social gaming by adding in Facebook and Twitter connectivity. You'll be able to share your scores with other gamers as well as brag about your kill count.

One of the best features of this game is the sound. Resolution Interactive really focused on developing the sound design for BioDefence. The explosions and music score do a superb job of thrusting gamers right inside this zombified world. If you're one of those people who must play music from your own iTunes library, you can do that as well.

BioDefense may not have a wide variety of monsters or weapons to choose from, but the game more than makes up for this caveat with relentless attacks that really push the difficulty levels of TD iPhone games. The vibrant sprites may get overlooked do to the larger playing area in this TD game, but you'll be too busy delighting in the massacre of hundreds of zombies to care.