I highly recommend Anomaly: Warzone Earth to anyone looking for a clever challenge!
Technically, the visuals and sound effects are quite competent and even impressive at times, and help make the experience more immersive.
The premise is simple and actually very much outdated at this point: aliens land on planet Earth and it's up to you to stop them. But as with all Tower Defense games (or in this case, Tower Offense), storytelling isn't really all that important, if at all. And while Anomaly's attempt at an ongoing story is somewhat praiseworthy, its real trump lies in the engaging and addictive gameplay presented to the player. There are only 6 types of enemies and 6 different (upgradeable) armored units the player can control in order to attain victory (and even these are only slowly unlocked as the story mode progresses), but don't let that fool you - the missions are interesting and surprisingly varied and even if you're pretty much always facing the same foes, each and every single approach to any given level feels almost always fresh and new.
The gameplay is a lot more quick paced than the early missions seem to indicate - the action starts slow, but gets hectic real fast. You'll soon find yourself immersed in tough battles, where fast thinking and swift decision making may be all that stand between you and defeat, especially in the harder difficulty settings. But not all takes place in real-time. A good preparation prior and at times even during your assignments is absolutely essential for the survival of your convoy, especially because you can only actually control the Field Commander; your units (the ones that actually do the fighting) won't stop for nothing and will always keep the same pace - planning the best possible route on your tactical map is key to success!
As long as you have enough money, you can approach every situation in a good number of ways, seen that you can buy (spawn) and sell units at will.
Aside from the story mode, a couple more bonus missions (which act pretty much as a survival mode) will be unlocked at certain key points during the single player campaign. These present simple objectives and increasingly larger and tougher enemies, and they're always fun.
The story mode isn't all that long, so I think the game would've benefited from some sort of free mode where the player would engage in battle against randomly generated enemies in equally randomly generated maps. However, the bonus missions and the 3 different difficulty levels available to the player are likely to keep him busy for quite a while.
If you like tactical planning and/or a good tower defense game, you'll more than likely very much enjoy Anomaly: Warzone Earth. Highly recommended to anyone looking for a clever challenge!