Alien invasions are as much a part of videogames as twitchy index fingers. But while the space marines get their share of limelight, we don't hear as much from the builders, engineers and strategists. Sanctum 2 redresses the balance, offering a game that's equal parts firefight and fortification.

On paper, mixing a tower defence game with a first-person shooter seems risky: Will the strategy element be smart enough? Will the combat be sufficiently thrilling? When game developers try to please everyone they often end up pleasing no one. But in the case of Sanctum 2, Reverb has walked the tightrope with aplomb.

The object of the game is to defend an area from swarms of insect aliens, from creepy crawlies to giant monsters. Tower defence is first: You have a few moments before each wave arrives to fortify your position with walls and different automated guns and mines. Position these blocks correctly and you create a maze – manipulating and funnelling your enemies into a line of fire.

This prep work is followed by a familiar, fast-paced, first-person shooter as you blast away at the invading horde to defend a precious oxygen source.

You can choose between different kinds of defence: do you want lots of small guns or a few big ones; thick walls, or a longer, meandering tunnel? You have a choice of soldiers, with unique weapon styles. There’s also the option of co-operative multiplayer games. It’s well-presented: No matter how chaotic the game becomes, it’s always clear how much damage your base is taking and where the enemies are pouring in from.

Sanctum 2 is not a rich narrative, but it's a satisfying, fun game with intense action and logical, rewarding strategy.