Mauritz

 was a man who created a monastery peopled by several versions of himself.

Mauritz wanted a place in which he could learn. He created the monastery as a five-dimensional space mapped into three dimensions. The citadel did not merely allow access to the future, it allowed access to all possible futures.

For a long time, Mauritz was alone with his other selves. When the Fourth Doctor and Adric visited him, several versions of them remained at the monastery while other versions left in the TARDIS. (PROSE: Mauritz)