Ctesiphon

Her long awaited vacation cut short by an orbital invasion, bounty hunter Ethelyn Dax takes on a whole army of mercenaries and their robotic auxiliaries to reconquer the resort city of Ctesiphon.

Blast your way through seven levels and a climactic bossfight with an old rival, making use of an arsenal of varied weapons to take on a multitude of enemies. Level design is open-planned, encouraging you to flank enemies, find alternate paths, and scout for secrets.

Made from scratch over the course of nearly a decade by a single developer on Linux using C/C++, OpenGL, and open-source Quake mapping tools (with the exception of the key art, music, and a couple of textures). The entire game, the engine it runs on, and the tool stack for all the assets went through multiple rewrites and long hiatuses before finally taking its final form in the last couple years of development.