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 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 Doctor was at Arcadia, fighting on the front lines. (COMIC: The Forgotten) 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) When he mentioned the battle to Rose Tyler in his tenth incarnation, he still hadn't mentally recovered. (TV: Doomsday)

Behind the scenes

 * Arcadia is also a region in the south of Greece, on Earth. It was known in classical times as the home of the god Pan and was touted in Renaissance art as a peaceful, bucolic wilderness.
 * 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. Amy Pond mentions that the Eleventh Doctor took her to a place called Arcadia (TV: Vincent and the Doctor), although in hindsight, this cannot have been the Arcadia on Gallifrey, and was more likely the region on Earth or another location with the same name.