Defender's Quest: Valley Of The Forgotten is one of the best games I've played in the last half-decade: a massively replayable fusion of RPG and tower defence. Better yet, it's also incredibly cheap on GOG, saving you £8 versus The Humble Store!
I awarded a 9/10 score in our Defender's Quest Review and the game has only improved since then, offering new features and tweaks, plus a massive free Gold upgrade with new sprites, and an HD version on the way at no extra cost. Remember to check your Steam backlog, though, because it did feature in a Humble Bundle back in September.