Old Greyhawk Castle, Level 13

 Bottom Level / Zagyg / Invisible. 

Contained an inescapable slide which took the players clear through to China. [3]

Invisible Stalkers and other minions of the secret master of the the castle herded him to the center, where a magically enabled 'slide' carried Robilar down through the earth and out again on the other side of the world, exiting in a strange temple in a land very much like Cathay (China), where nobody could understand him, and from whence he would hav to adventure his way back to Greyhawk over land and sea.  Before being launched, though, as a fighter Robilar had found +3 armor, shield, and sword; and as he careened down that chute to elsewhere he saw none other than Zagyg the Mad Archmage waving bye-bye to him from a sphere of force above, granting with that gesture one whole experience level to Robilar for his accomplishment. [15]

Room with Reward Statues for reaching the bottom; Fighter Statue with Boots of Elvenkind, +3 sword, +3 shield and +2 plate mail. Also there, a staff of wizardry, snake staff on statues of Clerics or Mages if you were of that class. 

When i was running the campaign alone, the dungeons of Castle Greyhawk wree only 13 levels deep. On the 13th was Zagig himself--he observed what went on above, restocked, etc. When a character got down to his level there was no going back. The one managing that was given an appropriate reward then sent on a giant, one-way slide clear through to the other side of the world, a place akin to China;) They had only what they carried at the time.
finding the lowest level was very difficult. Rob, playing Robilar solo, delved into the dungeon, made it. Ernie, noting Rog's absence from adventuring with the party, sent Tenser on a solo quest to discover Robilar's whereabouts. He managed to follow a similar path, and made level 13. Then Terry Kuntz noted both of his usual companions were nopt available to play, went forth with Terik, and made the lowest lever successfully. These PCs were around 10th level at this time. Rob never mapped, and Ernie didn't either when he went exploring with Tenser, so there was no cheating. Can't say how they managed it, but all three did it in succession. Each then solo-adventured back overland syccessfully via different routes.
No other players in the group managed that. About a month after all that Rob and I combined out castles, and Greyhawk Castle's dungeons grew massively, from about 20 levels total, 13 deep, to over 40, going down to about 28 levels. #1307

Castle Greyhawk had to have such a figure behind it. Otherwise, how could one explain all the strange and near-impossible (even in a magic-active universe)? So the advent of Zagig corresponded to the development of the castle-dungeons complex early in 1973 used in my Greyhawk campaign. Zagig put in a cameo appearance when the adventurers managed to plumb the utmost depths #1319

In the bottom-most level of the Castle Greyhawk dungeons lies a massive gate of horn, beyond which is a small dimension. Within lie weird environments including a savannah of mobile plants that attack interlopers, a great "Egyptian" tomb, a desert of golden dust where north equals south and south north, a frozen wasteland and a huge lake where a large Slimey Horror possessing huge claws and the ability to reduce the unlucky to yellowish clay if the thing's mucous is breathed on them when they are in the water. It is also the demesne of a mighty storm giant. Fabulous treasures can be won in each of these lands, but the mightiest of them are the fabled Teeth of the Barkash Nour. Those passing through the Gate of Horn at the very roots of the dungeon of Castle Greyhawk have but twenty four passing hours to locate the teeth and retrieve them all, else be expelled from the weird plane, never permitted to enter again...

Hoist by my own petard! These three, separately, had attained the nadir (pinnacle in terms of success) of the dungeons, and thanks to Zagig were sent "clean through the earth" to a distant land. Having sown the seeds of my own undoing, how could I complain? So I was faced with major works of improvisation as one after another of these PCs (for the record Robilar, Tenser, and Terik) made their separate ways around the globe, seeking to reunite as they quested for their own homeland. While I was  pleased with their enjoyment of the adventuring fare, it was less palatable to the DM. As it happened, each character decided on a different route for their trek. My capacity to invent interesting, different, and exciting material on the spot was stretched to the limit by a long series of one-character adventures, and I determined never to go through such a trial again. So as the triumphant trio of PCs who had penetrated to the lowest level of Castle Greyhawk and survived being sent as far from there as the world allowed received their well-earned laurels from their less enterprising fellows, as DM, more world building was feverishly in progress.

[References: see Greyhawk References]