Tower Defense Games

Does anyone know of any good tower defense games?  I know there are a lot of small silly ones but I'm looking for one with a serious tone and level progression.  Any suggestions would be appreciated

Defense Grid: The Awakening

That looks, awesome! I will probably buy it

Agreed Defense grid is a good tower defence game

3 comments....only one suggestion... lol what platform you looking for? PC, android IOS? Orcs must die is pretty good although I don't know if you would totally consider it a tower defense but its really good. Kingdom Rush and Anomaly warzone earth are both really good for IOS (and android I think?) Anomaly is like a reverse tower defense where you send military units through a city infested with alien towers. 

I didn't mention platform because I thought it would be redundant to specify PC since I posted in the PC gaming section lol

revenge of the titans is pretty fun