Fall of Arcadia

The Fall of Arcadia was a Gallifreyan city and site of one of the battles of the Last Great Time War. It was thought to be "the safest place on Gallifrey", because it was protected by 400 sky trenches. However, a massive fleet of Daleks invaded from above and began to exterminate the Time Lord soldiers protecting the trenches. (TV: The Last Day)

The 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)

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