Possibly a dome city built around the present-day city of Moscow, Mosk is a dome that was attacked and destroyed (at least in part) by Romdeau. Vincent Law believed he was from the City since his memories of the past only start with him leaving the dome with the other survivors and emigrating to Romdeau. But in fact, it was the place where he (as Ergo Proxy) and Monad Proxy conspired to remove his memories and allow him to live anonymously as a human.
The wreckage of several Romdeau AutoReivs (security AutoReivs which carry a human rider) suggests that Mosk was not destroyed by long-range bombardment but by conventional attack (the long range bombardment by the Rapture, initiated by Raul Creed, came later). Mosk is also home to a chamber in which Monad Proxy left the memories of Ergo Proxy encoded upon the walls, as well as a duplicate pendant that might have been Monad's, in the care of the AutoReiv called Amnesia.
Mosk was destroyed as revenge by Romdeau's regent, Donov Mayer. Since Proxies power the WombSys, he felt abandoned/betrayed when Ergo left them to go to Mosk permanently. Donov destroyed Mosk in an attempt to force Ergo Proxy back to Romdeau. It can be speculated that Re-L Mayer was adopted by Donov because she was cloned from a Proxy and was as close as Donov was going to get to regaining his beloved Ergo Proxy.