Orcs Must Die! is a brilliant tower defence hybrid that provides hours of singleplayer shenanigans, and I'd heartily recommend it. Matt certainly does. Though its sequel is a superior cooperative experience, the original Orcs Must Die still stands tall as an engrossing and hectic singleplayer shenanigan - and £1.75 is an utter bargain. You can also grab the GOTY edition with extra DLC for £2.24.
There's an added bonus. You'll get a Steam code, so you can use this to unlock ten extra levels in Orcs Must Die! 2.