Old Greyhawk Castle, Level 2

 Cells/Dungeons/Cellars/Deep Cellars. 

Had two unusual items, a Nixie pool and a fountain of snakes [3]. A pond of never-ending snakes. [20, p. 63]

Covered pits appear, and could be placed in rooms and chambers. [16 / 294]

Giant lizard (with gems in his innards). Wights in a dungeon cell area. [?]
(Here the episode of "it was all a dream" with Robilar could have happened, or maybe on level three). 

It was quite possible to journey downward to the bottom level by an insidious series of slanting passages which began on the second level, but the likelihood of following such a route unknowingly didn't become too great until the seventh or eight level. [3] 

Robilar was trapped at 2nd level near abandonned cells by two wights [41]

Also here is a transporter room 20x20 to a lower level (likely level 4) [14].

In the home campaign [12], likely because they already were level 4, the players encountered: six gnolls, then a troll, and eventually a giant lizard, a human warrior and a dozen orcs that were his servants, winning 3,000 gp in gold and gems, a wand of paralysis, a snake staff, and a +2 dagger.

[References: see Greyhawk References]