Fortress Island

Fortress Island was a small island located on Earth in the English Channel. After being tried and found guilty of his crimes, was sentenced to be incarcerated in the prison on Fortress Island for life. A few months later, the Third Doctor visited him. He 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 John Hart's map.