Fortress Island

Fortress Island was a small island located on Earth in the English Channel.

History
was incarcerated in the prison on Fortress Island "forever", and was later visited by the Third Doctor. The Master escaped soon afterwards with the assistance of prison governor George Trenchard. (TV: The Sea Devils)

After it ceased to be a prison, it became a retirement home for the compromised ex-UNIT operatives, who would pose a security risk. (AUDIO: The Screaming Skull)

Behind the scenes

 * Fortress Island isn't named in the on-screen dialogue, but appears on the admiralty chart Trenchard gives to the Master and the chart Captain Hart gives to Commander Ridgeway.