He aided Karlax in initiating a mind probe on Cinder. However, he did not like this and hid Cinder behind Rassilon's portrait so she could recover, showing he had scruples and limits despite suggesting using the Tear of Isha to stop the Daleks from dematerialising Gallifrey, winning the Time War. When the War Doctor tried to escape Gallifrey with Cinder, he found the codes had been changed, meaning his TARDIS was trapped. The Castellan tried to persuade him to surrender, but the Doctor told him he wouldn't be able to have the deaths of the people of the Tantalus Spiral on his conscience. The Castellan transmitted the codes, enabling the Doctor's TARDIS to dematerialise.
Karlax suspected that the Castellan had helped the Doctor escape and told Rassilon that he would investigate the matter. (PROSE: Engines of War)