Old Greyhawk Castle, Level 7

Maze

Centered around a circular labyrinth and a street of masses of ogres. [3]

While the names above are established, the order of this and the remaining is not well resolved, the name Maze is also given for level 13, and sometimes for earlier levels.

The Face in the Floor 

"Erac's Cousin [...] came upon a beautiful face that cried golden tears. The face told the adventurers the tale of his imprisonment and the heroic deeds required to release him. The adventurers agreed to recover The Urn of Moon Dust from a group of werebears. [They] successfully recovered the urn, and [...] sprinkled the moon dust on the weeping visage. The face was actually the demon prince of deception, Fraz-Urb'luu, who had been imprisoned by the mad arch-mage Zagyg centuries earlier. The completion of the quest resulted in his release. The demon then whisked himself and the adventurers back to his own plane where strange forces drained the magic from all [their] items. Fraz-Urb'luu quickly subdued the stranded adventurers and they suffered unspeakable tortures at his hands before they eventually managed to escape." "Actually the face duped our duet into pouring a potion onto it. The potion was held by a very large and very good aligned wearbear, which for some reason our duet didn't wonder about."

The doughty explorers made their way quickly through some upper levels of the dungeons and came upon one where the main encounter was with the bas-relief head of a magically chained demon prince, Fraz-urb-luu. Upon entering the place, the demon’s power was sufficient to prevent the paladin from detecting evil.  [...] off the two stalwarts went to an 8th-level dungeon maze at the center of which a werebear guarded a special scroll that would free the demon lord from his confinement [...] Eylerach read the scroll, and the freed Demon Lord carried both PCs with him to the Abyss, where their magical swords (2 vorpal swords and a holy avenger +5 that Gary wanted out of the campaign) became useless. [16/320]

This is a good example of why giving to much treasure and then taking it away sucks. Ernie was aware of the trap, but continued on to play in character  I don’t think Ernie has forgiven me for the loss of Erac’s blades, and he had just cause to be upset, Gary wrote 30 years later. Clearly one should learn from this. 

The location of the face on the 7th is not sure, as it is not clearly stated if the PCs went down to the 8th, but the intro says they went through some upper levels - if the dungeon has 13 levels, with the 13th being a special exit level, then upper levels would be 1-4, mid-levels would be 5-8, and lower levels would be 9-12. There would be no point to mention they went to the 8th, if they already were there, to it must have been above the 8th. So it could be anywhere between level 5-7. But level 6 already has strong unique encounters with both wereboar-maze and was the "black dragon level" that players avoided, and if it was level 5, this would be gap of full three levels, it seems a bit too far apart.

Well of Shadows is bottoming out here (but was not part of the Original Castle, only of the home campaign).

[References: see Greyhawk References]