Atlantis lockout device
A device which 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.
The lockout device may only be keyed to certain ranking Lantean military officers, or it might be configured to read the Ancient 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. The device is probably designed to be used in specific situations such as footholds.
It is possible all Ancient city ships possess this safeguard.
The Return, Part 1 - Eager to retake Atlantis for what is left of the Ancient people, Captain Helia uses the Atlantis lockout device.