Getting into RPGs isn't as easy as FPS or other popular genres, but since you're a fan of Kingdom Rush (Ironhead was a good name but nobody ever used it) you'll enjoy the strategic thinking. One thing to know about RPGs is that your character the first time around is going to stink. Fortunately, playing again means that you know all of the game's workings from the start, so you can build an OP character and feel like a God.
I'd recommend any games that bring in RPG elements but aren't strictly RPG, like any DMC games.