The three scallop shaped areas around the perimeter look like exhaust vents from the center. I'd guess a silo except there are almost no defensive sites (AAA, missiles, etc) around this area.
A few years back a poster spoke of a NK mountain hollowed out for a hidden launch platform for ICBMs. I poopooed the idea at he time, but this looks exactly as he/she described it. I'll try to track down the discussion.
This web site claims it is a hardened SA-3 SAM site. He says there are two of these near Pyongyang. This is the one SE of the city. He includes a map that shows roughly where the other is on the NW side of the city.
Check 2.7km NW. Sure looks like an 8 position SAM site with trenches dug to connect. Beside the (old?) Mirim Airfield. No aircraft - but a large neatly parked collection of vehicles on western part.
http://www.militaryphotos.net/forums/showthread.php?t=128528