Fall of Arcadia

The Fall of Arcadia was a battle fought on the last day of the Last Great Time War.

The city of Arcadia was thought to be "the safest place on Gallifrey", because it was protected by 400 sky trenches. Despite this, a massive Dalek force of millions of Dalek fighter pods and Dalek flying saucers managed to penetrate the trenches and swarmed over the city, exterminating Time Lord soldiers and civilians alike. (TV: The Last Day, The Day of the Doctor)

The War Doctor was at Arcadia, fighting on the front lines. Witnessing the devastation, he blasted the words "NO MORE" into a wall as a message to both the Daleks and the Time Lords, and departed to steal the Moment with the intention of ending the war once and for all. (TV: The Day of the Doctor)

Later references
When he mentioned the battle to the Cult of Skaro in his tenth incarnation, he still hadn't mentally recovered. (TV: Doomsday)

The Fall of Arcadia was depicted in the painting Gallifrey Falls No More. (TV: The Day of the Doctor)

Behind the scenes

 * Prior to the The Day of the Doctor, Arcadia was presumed to be a planet that was devastated by the Time War. Specifically, the Doctor Who: The Encyclopedia states that the Doctor watched as the Daleks fled from the planet.