The Forbidden Fortress, also known as the Fortress of the Astronomican is the base of the Adeptus Astronomica, incorporating the Chamber of the Astronomican and the Astronomican itself. It is a remote and secret complex on Terra, built within the Himalazia mountain range. To outside appearances the mountains remain unchanged by the hand of man but in fact the Fortress extends deep down into the rock and throughout the entire mountain range. The Chamber of the Astronomican is a massive spherical chamber located within a single mountain, the peak sculpted to form the upper half of the sphere of the Chamber. Uninvited access to the Fortress is not permitted - even members of the Adeptus Arbites and the Inquisition must seek permission before they can enter.
Around the Fortress are vast city-sized emitters that transmit the power of the Astronomican across the Imperium at large. Collectively, these constructs are similar in size to the Imperial Palace itself. All of this vast network of ancient but degrading technology is continually maintained by Tech-Priests of the Mechanicum and its power conduits lead all the way back to the Golden Throne within the Imperial Palace.