Old Greyhawk Castle, Level 3

 This is the third level of the Old Greyhawk Castle, named THE DUNGEONS, redrawn for clarity.

The original map is somewhat blurry, so there are several areas in the redrawn map that could look somewhat different. Here is the original:

Level 3 featured a torture chamber and many small cells and prison rooms. [3]

A magical laboratory that's difficult to find exists near the center of dungeon level 3.  [16/287]

The hexagonal room in the upper left likely was the torture chamber, East of it are the cells. 

The "Secret Laboratory near the center of the map" could be the section that includes the diamond shaped room.

More dangerous pits began to appear: spiked pits, pits 20' or 30' deep, trapdoors that close upon the pit victim, poison on the spikes, black pudding, yellow mold, etc. at the bottom.  [16/294]

It's not quite clear to me if the darker corridors were intended to show lower ground, or overlap, or if they just end at each other. I think slopes and overlap is more interesting but harder to make work logically. 

This map also shows that Gary remodeled / re-edited, with additional rooms added at the very bottom, and in some areas, darker shade of erased solid stone sections showing. 

The position of the central Staircase and the round room at the upper right match to level one, which makes me think that room also was a shaft that traversed multiple levels.

I'm not sure if the greenish areas on the photo had any meaning. Could be areas of poison gas.  Or based on [19], a sloping corridor "Sloping ... green". Notably, the Dungeon Geomorphs also suggest using green to denote sloping passages -- or gas: Color will be helpful nothing such things as passageways which are slanted so as to lead upwards or downwards (perhaps green, with the letters "U" and "D", with appropriately pointing arrows to indicate the direction of the slant), gas areas, and special doors (non-resisting, wizard locked, intelligent, etc.) [21]. Normally sloping passages would aim to confuse the characters as to what level they are on. This is not the case here, it is just a short, dead end passage, although it seems to contain an error indicating sloping down, matching the one in the middle of the top of the page. Maybe this indicates a subterrean passage/connection not shown, to confuse mapping?

Are the yellow areas teleporters, as on level one? "Teleporter rooms are delineated by yellow" [19]: The key to Gygax's binder map of level 1 shows a circle filled in with yellow with "Transporter" written next to it. OD&D Vol 3 uses "transporter" in the sample level as name for a teleporter, so the two seem to mean the same. The Dungeon Geomorphs suggest "putting in areas where those who enter are teleported to a similar spot elsewhere", which appears to be what Gygax used for the binder map level one (the two yellow-colored areas are similarly L-shaped), but not here.

We again encounter the weird bisected squares, that were in front of one of the stairs down in triplicate on level 1, this time sectioning off an area. Covered pits, maybe? 

The "Well of Darkness", from this blog post is highly likely the "Well of Shadow" and was a later feature not present in the original castle (a pity really, the play reports from his refurbished home game in 2007 sounded awesome): Jon Creffield and I did a pretty deadly and most unusual dungeon crawl module back in 1999, The Well of Shadows. He is working on re-creating the lost level maps for it, and I hope to turn the ms. over to TLG by the end of the year. [35]

Obmi

Obmi first encountered here, with gnoll archer servitors; Obmi had boots of speed and dwarven thrower; gnolls and Obmi had a repulsion projector ray (10'-wide beam, 90' range). [16/287]

Repulsion ray included both a reversal of course and a temporary illusion that the subjects were still moving towards the dwarf. After a round of enforced retrograde the effect ended, and the party was back where it started from. [?]

Obmi appeared in may campaign as a vile dwarf who worked with gnolls (see other thread on same on the boards). Worse, Obmi has a machine-like device that would cause the opponents to move backwards instead of forwards when they tried to attack him and his band in their fourth level dungeon abode. He had his throwing hammer and speed boots too, so that he really enraged the players. Eventually they managed to kill all his gnolls, his reverse motion machine was destroyed by spell attacks, and so Obmi ran off to return in the G series modules. #953

 Elevator Room. The elevator on level 3 (EGG/GrhCastle 1). Seems simple, but the idea is very complex for a supposed medieval society.  And it wasn’t magical. [RJK]

[References: see Greyhawk References]