Still haven't had your fill of tower defense games on the iPhone?  Here's another one to quench your thirst for slaughtering waves upon waves of enemy vehicles: iBomber Defense.

If the name sounds familiar, that's because this is a spin-off of the classic iBomber games.  Instead of sitting in a cockpit shooting at enemies, though, you get a bird's eye view of the landscape, protecting your towers from approaching enemies.

The game sticks to the staple tower defense formula, so don't expect much of the fancy twists that have become part of modern titles in the genre.  It's a good thing, too, since developer Cobra Mobile poured all their attention into producing an excellent gaming interface and a challenging campaign.

A total of 22 levels are available in iBomber Defense's Campaign mode, spanning Africa, Europe and Asia.  There's also a Quick Play option with a separate set of achievement goals and three levels of difficulty.

Some tower defense games try to wow players with variety in weapons and towers.  This one keeps things under control, making gameplay much more focused (and, to me, more satisfying).  Plus, the backdrops and graphics are stunning, to say the least, making for a real visual treat.

In many ways, iBomber Defense is a traditionalist in the evolving genre of tower defense.  Rather than focus on bells and whistles, it tightens up nearly every imaginable aspect of the experience, producing a challenging and entertaining title that requires good forward thinking and well-timed reactions.  And, yes, it supports Game Center, too.  You can get it from the App Store for $2.99.

[iBomber Defense]

Tower defense plus zombies isn't exactly an original concept. BioDefense: Zombie Outbreak, however, dares to break the mold, bringing an original take on the genre that's a little different from2 anything we've seen on the iPhone before.

In the game, you are tasked with defending your headquarters from hordes of zombies coming at it from every direction. That's right, they're coming from all angles, as your territory sits right in the middle of the playing field. More than a simple blast-em-all, though, it mixes open-map tower defense with element of real-time strategy, turning a game that delivers just a little bit more on depth than your usual mobile title under the same category.

First problem: everything around you is dark, so you can't make out anything beyond your own headquarters. The only time you'll ever catch the zombies this way is when they're already right in your face. To afford yourself a bit more vision, you'll need to set up a network of floodlights, which you'll also need to defend because the zombies can take them out too.

To build floodlights, generators (you need power), other towers (such as missile launchers) and defense systems, you need "toxium," which you can mine by setting up collectors around the playing field. They're limited, though, so you'll have to spend wisely. Many times, you'll use up all your currency to set up your defense only to realize it's not adequate and there's nothing else you can do.

Sounds basic? Yes, it does. However, maintaining the balance between the limited toxium, the power and your defense against the ever-increasing zombie onslaught offers a formidable challenge - one that can leave you playing this game for days on end. We're not kidding.

Priced at $2.99, BioDefense: Zombie Outbreak has possibly the most addictive tower defense survival mode I've ever played on any mobile platform. You can't help but love it.

[Resolution]