The Metal Slug series, well-known for its gorgeous, pixelated animations, wild shoot 'em up action and fierce difficulty, seems like a perfect fit to spin off into a tower defense game. Wait, no, these two things don't go together at all. That didn't stop the brave lunatics over at SNK Playmore from patching this Frankensteinian creation together and creating something wholly unique.

In Metal Slug Defense, the evil General Badguy (or whatever his actual, irrelevant name is) has returned to reign badguy terror on the world, and it's up to the Metal Slug team to take him down. To do so you'll jet around the globe, stopping his military campaigns one level at a time. Metal Slug Defense takes the basic shell of tower defense gameplay and fills it with a creamy, two-dimensional real-time strategy center. During each stage you and your opponent have stationary bases at the opposite ends of the screen, and use progressively accumulating resources to send wave after wave of different units at each other. While early levels only allow you basic soldiers, like the aptly-named soldier, after beating a few areas you'll unlock classic Metal Slug heroes like Eri, Marco, Fio and Tarma.

These heroes, like all of your units, come with special timed abilities that can be activated once each of their special ability gauges fills. Fio uses a rocket launcher, which is good for individual, strong opponents; Eri has a laser gun great for clearing long lines of weaker foes; Marco has an all-purpose machine gun. Even the lowly soldier has a grenade he can throw for high damage, though you'll have to time his grenade carefully or you'll bomb naught but dirt. Thanks to these special abilities (and the different types of sidearms wielded by each unit), your army feels diverse and useful. Clever utilization of your resources will inevitably lead to victory, even when you're facing down a giant crabtank— a beast equal parts crab and tank.

Whether you win or fail a level, you'll accumulate MSP, a currency used to level up your individual units, and your base overall. It's here where Metal Slug Defense does a great job of hooking you into playing the next level, as you'll continually earn new, base-wide upgrades like faster resource generation or increased damage on your all-powerful Metal Slug attack, as well as focusing on powering up the units you find most useful. Plus, at the end of each area you acquire a new unit, furthering your combat options. Eventually you'll have so many units that you'll have to choose which ones you're bringing into battle, and as they're all so fun to play and colorful to look at, it becomes a tough choice.

While Metal Slug Defense's gameplay manages to be surprisingly fun, one thing that's not-so fun is its pricing. With this being a free-to-play title developers SNK Playmore needed to find a way to make money somewhere, and they do so via the in-game shop. There are all sorts of in-game units you can purchase using "medals,” which are primarily acquired via in-app purchases, and power-ups/MSP bundles you can snag to buy your way through the tougher levels. You also have to be mindful of Sortie Points, points you use up with each attempt of a mission. Once you hit zero, you can't play again until you've waited for them to refill or spent the real money necessary to continue. None of it is especially underhanded, but if you lose the same level repeatedly and watch your sortie points plummet, you'll wonder if it's because the developers chose to make the level challenging, or wanting to coerce you into busting out the credit card to get back into the action more quickly.

A bizarre marriage of tower defense, real-time strategy and 2D shoot 'em up action, Metal Slug Defense is like Kingdom Hearts— a blend of disparate things that somehow manages to work. While its free-to-play trappings can be a little tiresome, they're relatively innocuous... and ad-free! Patient gamers can easily ignore MSD's gentle offers to spend real money, and instead freely enjoy a colorful, addictive real-time strategy game like no other.

This review is based on a download of Metal Slug Defense for iOS.

App Store Link: Metal Slug Defense for iPhone and iPad | By SNK Playmore I Price: Free | Version: 1.0 | 30.2 MB| Rating 12+

8.0 out of 10 arcade sushi rating