Estragil was a city on Gallifrey.
During the Time War, time distortions happened there. When the War Doctor confronted Rassilon about this, he sent the Doctor along with Asmuth to investigate and destroy the cause of the distortions.
While investigating the city, the Doctor and the four versions of Asmuth faced Reapers and several Daleks, including the Skaro Degradations. (NOTVALID: Bigger on the Inside)