By now, most gamers are familiar with the concept of tower defense games: defend your base by placing various turrets alongside a predefined path that will soon be swarming with invaders. Anomaly: Warzone Earth takes that well-explored formula and turns it on its head. Instead of building defensive towers, players take command of a squad of attackers attempting to repel an alien invasion.

Set in the near-future, the game begins in Baghdad when an alien ship crash lands in the middle of the city and encases itself in a giant dome of energy, known as the Anomaly. Before all is said and done, players will also travel to battle aliens in Tokyo as well.

Players take on the role of the leader of Squad 14, known as the Commander, whose job it is to provide support to his well-armed convoy. The commander moves independently of the convoy, and is able to throw down a number of different power-ups (healing, smoke-screen, decoy, and air-strike) to support his team in the heat of battle. The inclusion of the Commander adds some really interesting dynamics to the action, since you're not simply sitting back and watching the team proceed, hoping you plotted the right route. A few well placed power-ups in the heat of battle are very often the difference between victory and defeat.

The gameplay in Anomaly: Warzone Earth is broken up between real-time action on the ground, and a strategic view that can be accessed at any time and used to adjust the convoy's route on-the-fly, or purchase and upgrade more units. The convoys themselves are limited to six total units, but there are a variety of unit-types to consider. For example, APCs are cheap and have reasonable armor, but have weak firepower, whereas the missle-hurling Crawlers pack a bigger offensive punch at the expense of weaker defenses. More units are made available as the single-player campaign progresses, including tanks, mobile shield-generators that give extra protection to adjacent vehicles, and supply-vehicles that periodically generate power-ups for the commander.

At the start of a typical mission, players purchase a couple of units to start their convoy using limited starting funds, but can also obtain more cash by visiting certain points on the map, or by destroying enemies. The order of the units in the convoy can also be changed at any time, in case a certain situation requires heavier armor up front, or better firepower.

Enemy units are also well varied, and require specific tactics to confront effectively, and create some interesting challenges when different tower-types must be battled simultaneously. Beyond the standard enemy towers, there are other tower types like ones that use area-of-affect attacks, and ones that cause your allies to temporarily turn on the Commander. There are even some incredibly devious towers that will sap the energy of power-ups deployed within its range and use the energy to rebuild nearby towers that have already been destroyed.

The mission objectives during the single-player campaign offer more variety than one might expect, often changing focus midway though. Sometimes, you'll just have to make your way to the next objective marker, but sometimes you'll also have specific targets that must be eliminated en route. Timed missions and escort missions also make an appearance, but both implementations are a lot more fun than they sound on paper. Developer 11Bit Studios has done a great job making sure that things don't get too repetitive. Despite a reasonably-long main-story experience, I was still hungry for more when it ended, largely due to the unexpected variety of missions.

The map sizes for each mission range from small, single-objective affairs to huge layouts filled with several win-requirements. Beyond that, geography also comes into play. Levels set in Baghdad feature streets that are more wide-open and less constrained, while levels set in Tokyo feature huge skyscrapers and much tighter, grid-like city streets. It's a clever mid-game switch that requires the player to think a little differently when planning their assault-paths.

Anomaly also includes two other modes that are unlocked by completing portions of the single-player campaign. Baghdad Mayhem puts players on a large map where they must destroy a series of ten progressively more heavily-guarded alien power generators. In both cases, completing objectives within each round's five-minute time limit (and destroying as many enemies as possible) will result in better positioning on the online leaderboards. Tokyo Raid is another progressive mode where players must make their way through eighteen enemy-fortified islands and take out an alien control center. In essence, they're two additional campaigns, without some of the cinematic bells and whistles from the main story mode.

If Anomaly: Warzone Earth has a downside, it's that some of the automatic checkpoints are positioned in a way that can rarely result in endless "reload-and-die" loops. In a couple of instances, after a checkpoint save, my convoy was headed into certain death with no way to choose an alternate route, and no cash to buy or upgrade units. It's by no means a deal-breaker, but did require me to restart a few missions from the beginning.

If you're looking for some strategic action of a different stripe, Anomaly: Warzone Earth is a great bet. Although I was initially a bit skeptical about how fun a "reverse-style tower defense" game would be in practice, Anomaly very handily won me over with its clever mission-types, unit variety, and flashy explosions.

Steam is also running a launch-sale at the moment, bringing the normal $9.99 price down to a measly $8.99. If it sounds interesting to you, there's really no good reason not to check it out.

[Anomaly: Warzone Earth review based on the PC version, provided by the developer. iOS versions of Anomaly: Warzone Earth are still in development.]