There is an objective in "Outpost" regarding an S.O.S. message from "Defense". Go to the first Nazi controlled building, where a Demon snatches a fleeing Nazi, and fight through the area until reaching the upper level of that building. Find a hallway with three tripwire mines and two Demons. Get past the mines, then check the right room at the end of the corridor to find a ramp that provides access to the rooftop. You will find a corpse with a Kalashnikov on the rooftop. The communications apparatus for the achievement can be found in the corner of the rooftop.