The Gate Must Stand

The Gate Must Stand's great central idea makes it definitely worth a look, even if it does get a little repetitive in the back half.

The Gate Must Stand is a passable tower defence game with reasonable gameplay and decent replay value, though it can be undermined by poor pacing, UI and item descriptions that need improvement. While not a bad game, it's also not great. Reasonably cheap on a launch offer, it will sufficiently pass time and be fine doing it.