The Castellan was responsible for matters relating to security on Gallifrey, and answered to Lord PresidentBorusa. He was on Gallifrey during Omega's attempt to harness the Arc of Infinity. He attempted to kill the Doctor to stop Omega bonding with him, but instead killed Hedin, who took the shot to save the Doctor and allow the bonding to take place. (TV: Arc of Infinity)
During the Death Zone crisis, the Castellan was among the High Council members who received the Master on Gallifrey to send him into the Zone. The Castellan gave the Master a transmat device to return to the Capitol. When the Doctor and the Master were ambushed by Cybermen, the Doctor used it to escape. Wondering how the Cybermen found him so quickly, the Doctor found a homing beacon inside the recall device, the one thing the Master would be certain keep on him. Borusa observed that the Castellan had given the homing beacon to the Master, to which the Castellan replied that this was the Doctor trying to take revenge on him.
President Borusa immediately ordered a search of the Castellan's rooms. This turned up a chest containing the Black Scrolls of Rassilon, which Borusa then destroyed. The President ordered the Castellan interrogated using the mind probe, much to the Castellan's distress. Moments later, the Castellan was shot and killed by Gallifreyan guards, apparently after trying to escape. In truth, Borusa had been behind the Death Zone crisis, and had framed the Castellan. (TV: The Five Doctors)
He was succeeded by Lowri. Meeting her on Gallifrey, the Doctor realised that he never did learn the late Castellan's name. (AUDIO: Time in Office)