Fortress Island

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

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