He was responsible for matters relating to security on Gallifrey, and answered to Lord PresidentBorusa. This Castellan 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 it.
President Borusa immediately ordered a search of the Castellan's rooms. This turned up a chest containing the Black Scrolls of Rassilon. Borusa ordered the Castellan interrogated using the mind probe, but the Castellan was soon shot and killed by Gallifreyan guards, apparently after trying to escape. In truth, Borusa was behind the Death Zone crisis, and had framed the Castellan. (TV: The Five Doctors)
It is possible that this Castellan is Kelner from The Invasion of Time, having regenerated. However, the Doctor's impression that he is blameless puts this theory under some strain in light of Kelner's actions in that particular story.
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