From publisher blurb: Abandoned for centuries, the tower of El-Arrin is reputed to house secrets and lost knowledge of an ancient empire. But almost no one who ventures there returns, and the few that have were quite mad. Myth makes the tower and its secrets older than the desert itself. Who knows what riches were left behind? Hidden Knowledge can be used as an adventure in any campaign world, or played as a sandbox in Hellebarde's default setting. Written and tested with Castles and Crusades RPG, notes are provided for conversion to other OSR systems. Hidden Knowledge can be used stand-alone or as part of the Exterris Adventure Tome AT-5 (AT-5 plus ONA-17 through ONA-20).