Siege of Centauri is an upcoming tower defense game set in the Ashes of the Singularity universe in which humanity's colony on Proxima Centauri falls under attack by endless hordes of alien forces. The only thing standing in their way is the Carpathia, a ship dispatched from Earth that can drop defensive emplacements from orbit.
Similar to the game on which it's based, Siege of Centauri will feature huge numbers of units—"tens of thousands" at a time, according to developer Stardock. With each successive wave defeated, the colony grows to a new part of the planet, and the aliens renew their attack with increased vigor.
"With our engine, there were so many new gameplay elements we could add. For instance, our engine supports hundreds of simultaneous light sources allowing us to design interesting choices for players when it comes to map vision," Stardock general manager Derek Paxton said.
"Similarly, by supporting so many units, we can make defense choices in different situations much more compelling as there is a big difference between choosing a weapon that eliminates one unit at a time versus one that is designed to take out hundreds at a time."
Siege of Centauri will release "soon" in early access for $10, which strikes me as pretty reasonable for a good tower defense game. (I have no idea if it will be good, but I like to be optimistic.) In the meantime, you can check out a few more screens and stuff at siegeofcentauri.com.