Very little is known of Lord Castillo. We know he is a former Lord of Borgeniss and according to the letter he is planning to destabilize the entire Kingdom of Borgeniss. The letter claims that Sir Novascoshek served beside Lord Castillo during the time of the war lords. It is possible, though unlikely, that Lord Castillo was the figure we saw on the hill commanding the destruction of the town. It seems more likely that these pillagers were snuck into the heart of Borgeniss with the help of Castillo and the figure was a lieutenant of sorts.
He is the uncle of the current Lord of Norgrain, Balbaroy whose father took the cloth. When asked the advisors to Balbaroy refused to explain what led to the removal of Castillo. It would seem that Castillo has a knack for trying to track down probably Tenants of the Wardens with his purchase of the sword in Simpletis.