Defender's Quest: Valley of the Forgotten

Defender's Quest is an RPG tower defense. Your "towers" are archers, mages, and warriors, and you can equip them, level up, and allocate skill points.

I got the game through a Humble Bundle, and it was fairly good.  The basic game isn't that long, but if you want to 100% it, it'll take a while (you have to play through the whole game and the whole New Game+ with only your six basic towers and without recruiting additional towers, and then you have to level up a whole additional batch to beat some of the extra levels).  

Some parts of the game weren't as ergonomic as I would have liked -- eg, you might find yourself rapidly clicking your lightning spell on enemies that are about to get to you, and when you want to re-allocate your skill points, you'll need to click a bunch.  The story also wasn't much to speak of.  As a tower defense, though, it's reasonable.