Ancient city ship
Perhaps the most impressive technological marvel known to have been created by the Ancients, the city ship is a snowflake-shaped behemoth, stretching kilometers in diameter. Capable of intergalactic travel via a powerful stardrive, one of these craft crossed the 3.5 million light year gulf between the Milky Way and Pegasus Galaxies.
Though Atlantis is believed to have been considered the crown jewel of Pegasus, she was not alone. Perhaps this suggests that Atlantis was the first in a line of city ships. What is certain is she was the central nexus for all Ancient activity in the Pegasus Galaxy, and therefore their last line of defense from the Wraith.
A second city ship was discovered by the Atlantis expedition on Eldred's planet, its vastness shrouded by the crust of planet, as tens of thousands of years of growth had buried the massive craft. This suggests that there may be more of the craft scattered elsewhere throughout the galaxy, or other galaxies where the Ancients traveled.
A third city ship was found to be home to the Asurans, who had constructed an even larger metropolis around it. After they learned that the humans from Earth now inhabited the city of their makers, the Asurans launched their city from the surface of Asuras and attacked Atlantis.
Rising, Part 1 - Several million years in the past, the Ancients launch Atlantis from the surface of the Earth and fly the city ship to a new home in the Pegasus Galaxy.
The Tower - Sheppard's unit discovers that Atlantis was indeed not alone. Its sister city is discovered on Eldred's planet.
Progeny - Sheppard's team finds another city ship on Asuras, surrounded by a vastly larger metropolis -- only to have the Asurans launch the ship and try to destroy Atlantis.
First Strike - Under an unstoppable attack by an Asuran satellite weapon, the team sinks the city to buy more time. But the only way to save Atlantis is to launch the city and seek a new home planet.
Adrift - The team works on one task after the next to desperately try and keep the city alive, drifting in deep space with little power remaining.
Lifeline - With a new Z.P.M. stolen from the Replicators, Atlantis lands in the ocean on its new home planet.
This Mortal Coil - The Replicators destroy a copy of Atlantis, a city ship in secret use by a rebellious sect.
Enemy At the Gate - Realizing the Wraith are about to attack Earth the team decide they have to fly the city home to help in the fight.