HROT

HROT is throwback first-person shooter styled similarly to Quake and set in Soviet-era Czechoslovakia. In it, some kind of disaster has just occurred and you fight gas-mask wearing soldiers and other bizarre enemies using a standard array of FPS weaponry. According to the developer, the levels are based on real-world locations. 

There are standard old-school FPS elements, like color-coded locked doors and keys, but then not-so-standard elements, like semi skimmed milk as a health item. While most of it is Quake-inspired, there are some parts reminiscent of Half-Life, such as a section with cogwheels to crank. 

In addition to the standard single-player mode, there is also an Endless mode where you fight off waves of enemies.