With any luck, you won't need this deal since you've probably taken advantage of Steam's outrageous £2.99 winter sale. But those heady days are over now, and we must make the most of what we've got: a £6.00 price that undercuts Steam's full price by a fiver. Orcs Must Die! is a brilliant tower defence hybrid that provides hours of singleplayer shenanigans, and I'd heartily recommend it. Matt certainly does.
If you prefer cooperative carnage and persistent RPG elements, however, you might be better off with Dungeon Defenders.