A device that allows Ancients to lock the city and its systems out of anyone else's control. Concealed directly beneath the front of the Stargate, any authorized personnel immediately arriving can, without warning, call upon and utilize the unit to protect the city. (SGA: "The Return, Part 1")
The lockout device may only be keyed to certain ranking military officers, or it might be configured to read the ATA gene itself. Unless put into use, control of the city falls to whoever manages to occupy it themselves. The technology is, perhaps, a secondary component to normal city operations.
It is unknown whether all city-ships possess this safeguard, or just Atlantis.