The First Siege of Atlantis
A final assault upon Ancient defenses in the Pegasus Galaxy after the Ancients lost the rest of their territory to the Wraith, at the conclusion of the Ancient-Wraith war some 10,000 years ago. A precise length of time has never been assigned to this siege, but according to Melia's holographic report the Wraith bombarded the [[Lantean System]] for many years.
Ancient transport ships were dispatched throughout the galaxy to recover what Ancients they could, but many were shot down before they could reach the perimeter of the city shield.
Little is known if the Ancients even bothered to fight back once only the Lantean System remained. After the Wraith had claimed the rest of Pegasus for themselves this seems to be futile. Saving lives would have been the remaining priority. After all who could be rescued were aboard the city Atlantis sank hundreds of feet down to the ocean floor, where a mechanism clamped the city into place.
Before I Sleep - Dr. Weir witnesses the first siege of Atlantis herself as she attempts to negotiate for permission to return to the future.
The Siege, Part 2 - Major John Sheppard explains the story of the Ancients in the Pegasus Galaxy with the unhappy ending in the Lantean System before the Ancients departed.