From the back of the book:
Imagine a world of knights in shining armor, mighty castles, and high
chivalry. In this book, you'll find a wealth of information designed to
allow any DM to create a campaign world based on feudal Europe. Sections
address life in a feudal culture, new Battlesystem rules for the
resolution of sieges, a quick resolution system for massive military
campaigns, and an assortment of generic castles to spark your
imagination. The Castle Guide is a must for any player or DM who's ever wanted to fight for a seat at the round table.