The Tersurons were a species that lived on the planet Tersurus.
The Tersurons were able to build castles as well as vast sewer systems which could take centuries to find the way out of. They appeared to do this before the discovery of fire. (NOTVALID: The Curse of Fatal Death)
The architect of Castle Tersurus was subject to various bribes by the Ninth Doctor and the Master to make the other fall into traps while anticipating that they would later bribe the architect to do the same.