Forest (The Velvet Dark)
Forest (The Velvet Dark), because the word "planet" is not used in the text and one could read the forest as being something stranger than a planet.
Talk about it here.
The Tremas Master was exiled to a forest by the Time Lords after they stripped his body from him, leaving him as only darkness maintained by his will.
Within the forest's gnarled trees was a castle, where the Master was intended to live out his meager existence, before his TARDIS found him and created the Watcher Master, jumping between the forest and Earth in order to lure the Fifth Doctor and steal one of his future regenerations. (PROSE: The Velvet Dark)
Behind the scenes
The forest resembles the forest prison of the Duke of Autumn seen in charity story It's Raining Again by Stewart Sheargold and Paul Magrs in Perfect Timing II. In that story, the Duke of Autumn was implied to be the Tremas Master.
