Castle by the Sea is a collection of six Basic Fantasy Role-Playing Game adventures centered around different castles. Each adventure, created by various contributors and edited by Chris Gonnerman, offers a unique setting and storyline suitable for player characters of varying levels. The adventures range in theme, from a mad king's abandoned castle to a sorcerer's mysterious conclave, and include detailed maps, monster statistics, and treasure descriptions for Game Masters. The document also includes the Open Game License and copyright information for the various contributors and their respective works. A significant portion of the text details the specific layouts and encounters within each castle level, providing comprehensive information for running each game.