Sanctum 2 is one of my favourite games of 2013: an FPS/tower defence hybrid that worked near-perfectly. Its four DLC packs added loads more maps and environments, which are available in an £8.99 season pass on top of buying the base game.
OR you could buy the whole lot for £4.75 from GamersGate. Savings ahoy!
If there are four of you, you can fudge an slightly cheaper deal by buying GamersGate's £9.00 four pack, then individually picking up the season pass. It works out at £4.50 each - though it's a 25p saving for extra hassle.