From the back cover:
Enter the infamous ruins of Castle Greyhawk, the most formidable and expansive dungeon on Oerth. Its mazes and catacombs have drawn hundreds of adventurers from around the Flanaess. They are intrigued by tales of its riches, undaunted by its reputation for death in countless unspeakable ways. Getting there is no trick -- it's a well-traveled path. The trick is getting in and out again with your own hide intact!
Greyhawk Ruins is a complete guide to the most famous dungeon in the WORLD OF GREYHAWK campaign setting. All you will need is the ADVANCED DUNGEONS & DRAGONS 2nd Edition role-playing game, nerves of steel, and the resolve to challenge the puzzles and powers the ruins have to offer!
Greyhawk Ruins is designed for five to seven characters of levels 2 to 15.