Black Forest is a rebel base used by... rebels...
It is heavily armed, and has enough supplies for around 500 people. Although a big rebel station, it is hard to reach and most rebels heading to the base will die before reaching it.If they arrive to the base, they will usually be accepted almost instantly, even if a former CP unit.
Unlike other bases, these are heavily guarded and even has alot of technologically advanced devices, such as weapons and resistant tanks.
After the Combine Invasion, survivors may have quickly fled to this abandoned Cold War base, and marked the start of the resistance once they were organized enough.
They were attacked by the Combine as a desperate attempt, and an Advisor killed Asya infront of the base. The Advisor was killed by Genova's forces. After the incident, all rebels abandoned the base and were relocated to Genova.