Solitude

 was a mountain on Gallifrey.

According to the Tenth Doctor, the Citadel of the Time Lords stood in the mountains of Solace and Solitude on the Continent of Wild Endeavour. (TV: The Sound of Drums)

In the Old Times, Presus fought Death's Messenger in the foothills of the mountains of Solace and Solitude for three days and nights. (PROSE: Blind Fury)