Mindustry is pretty fun. It's micromanagement base building mixed with tower defense. Don't let the simple visuals throw you off.
Dungeons 3 is sort of a tower defense, though it is more base building. Still a lot of fun, and as far as Dungeon Keeper clones go, it's not half bad (though I still recommend War for the Overworld more)
Orcs Must Die 1 and 2 is a good fantasy-based tower defense game. Build traps and stuff. I think the second game even has co-op :D
Infested Planet is cool, the sheer numbers you take on in that game is damn impressive. Think Aliens space marine squad versus a whole horde of aliens
They Are Billions is a brutal but super awesome zombie tower defense game. One of the best, but not for the faint of heart.
Anomaly: Warzone Earth is sort of the inverse of a tower defense game; in it, you escort a convoy along a set path through alien defense. You can deploy repair packs, determine convoy order, deploy attack vehicles. Pretty fun!
They Are Billions is a masterpiece in my view, and a true old school strategy game successor that has the courage to be punishing. Really recommend it. It's quite addictive with it's procedural nature.