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 Dalek fighter pods and Dalek flying saucers managed to penetrate the trenches, destroying the once-proud city, whilst the Time Lord soldiers fought back in desperation. (TV: The Last Day, The Day of the Doctor)
Bowships were used at Arcadia, but failed to prevent its fall. (PROSE: A Prologue)
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)
Legacy
When he mentioned being present at the battle to the Cult of Skaro in his tenth incarnation, the Doctor claimed he still hadn't mentally recovered. (TV: Doomsday)
The Fall of Arcadia was depicted in the Time Lord 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, Doctor Who: The Encyclopedia stated that the Doctor watched as the Daleks fled from the planet.
