Title given to the Chief of Security of the Time Lords of Gallifrey.  Castellan Spandrell helped the Fourth Doctor to prove his innocence in the murder of the President, and to defeat Goth and the Master (4P).  His successor, Castellan Kelner, was a coward who betrayed Gallifrey to the Vardans and the Sontarans (4Z).  When Omega threatened to take over the Fifth Doctor's body, the Castellan (no name provided) went along with the High Council's decision to terminate the Doctor.  He even tried to shoot him, although Hedin interposed himself and died as a result (6E).  The same Castellan was later framed by Borusa, who arranged for him to appear to be in possession of the Scrolls of Rassilon.  He was sentenced to being mind-probed, and was shot by one of the Chancellery Guards when he tried to escape (6K).