Fall of Arcadia

The Fall of Arcadia was one of the battles of the Last Great Time War. The Eighth Doctor was at Arcadia, fighting on the front lines, and he witnessed the fall. (COMIC: The Forgotten) When he mentioned it in his tenth incarnation, he still hadn't mentally recovered. (TV: Doomsday)

Arcadia was a city on Gallifrey. It was thought to be the safest place, because it was protected by 400 sky trenches (TV: The Last Day)

Behind the scenes
According to Doctor Who: The Encyclopedia, the Doctor watched as the Daleks fled from the planet.