The most "pure" TD game I've played recently was Tower Dominion. It's pretty good, though it does still have a few annoying bugs.

Aside from that I've also played ORX which is pretty much what you get if you mix Carcassonne with Tower Defense. I kinda suck at it though, boss enemies fuck my shit up.

Finally I've played 9 Kings, which isn't really a TD game I guess though it does have some TD elements. It's mostly about finding tower/unit synergies to exploit and breaking the game before it breaks you.