Sanctum, Coffee Stain Studios' first-person tower defence/shooter hybrid from 2010, is getting a sequel on May 15. As usual, pre-ordering nets you extra goodies, but in this case, you get something far better than an extra gun skin. Namely a whole spinoff game, which looks quite spiffy.
Super Sanctum TD, which can also be grabbed on Steam separately for $4, is a retro re-imagining of Sanctum featuring traditional tower defence gameplay (with a dash of variety in the form of an active skill system). It lets you use towers from both the first and second games, and provides a set of leaderboards to go with its levels, along with a large amount of achievements.
Super Sanctum TD [Steam]