In the spirit of Fortress Maximus starring in Autobot City: Earth, this is the Decepticons' side of the story - the Decepticon Siege Bunker. The Decepticons did not get hand outs from Earth's governments and so their stronghold is a lot more skeletal and modest than the opulence that is Autobot City: Earth.
The Decepticon Siege Bunker - Overlord
The Medical Bay
Deployment for scouting
Lord Giga departs the bunker
"Ready the energon cubes for transport."
"I am Megatron, leader of the Decepticons."
The weapons depot
"Reporting to Megatron: I found rocket fuel 10 miles west of here"
"Ready the shuttle for launch!"