From the publishers blurb:
The Scorched Citadel is an adventure module which can be used in fantasy or dying earth campaign settings that have a wasteland region or Mythic Underworld. The Citadel is designed as a sandbox setting in the old school sense; meaning it can be approached how the players or referee sees fit. With that in mind, players should be aware that combat and exploration can be deadly in this module. A minimum party of 5 players alongside some hired retainers is recommended. If they can face the dangers herein and survive, there are treasures and magic items that blur the line between magic and technology. The players may end up with something just as deadly to themselves as to their enemies. Within you will find details for five guilds, 5 dungeons, and a detailed map of the Citadel. Where possible, the layout of the book is such that flipping pages is minimised. This module requires Old-School Essentials Classic Fantasy or the classic Basic/Expert rules to play.