Mars Outpost is another colony on the planet Mars. It is in fact one of oldest colonies (older than Mars City) dating back to 2112.
Background
It is divided into areas including the Entrance, Sectors 1-6, and the Reactor, etc. Which branch off of the center known as Junction.[1]
It was largely cut off from Mars City and didn't hear all the details of the events on November 15, 2145. But it suffered its own incident, the Mars Incident, not long after.
The same Doom Marine (B.J. Blazkowicz) apparently stopped both incursions. The Marine had been dispatched apparently from Earth in response to a distress call from Union Aerospace Corporation's research facility on Mars. The base was under attack by an unknown force and his mission was to acquire intelligence and neutralize the threat. Once his insertion was complete ordered to rendezvou with the other Marines at Junction for further instructions. He was told to expect heavy resistance.
Behind the scenes
In-game in Doom RPG this location is described as the Mars research facility, or the cloning facility, the medical facility, or the outpost. It is also referred to as the Biological Research Facility.
The Doom RPG website connected the game to both classic Dooms and Doom 3. And Doom II RPG had links to Doom: Resurrection IOS game which also had ties to Doom 3. Through Sam.