From the back of the book:
This supplement is devoted to that chief most of heroic requirements: setting up a base of operations that may provide an adequate defense against foes and be a meeting place for other heroes. Many heroes, such as Spider-Man and Daredevil, operate out of their homes, while most groups, such as the Avengers and Alpha Flight, have their own separate headquarters.
This supplement expands the rules for building headquarters featured in the Advanced Player’s Guide and provides additional options for Players and Judges when building their base.
It also features 30 pages of maps covering all sorts of headquarters, and two FULL-SIZE maps covering the classic Defenders Long-Island headquarters and Dr. Strange’s Sanctum Sanctorum!