Old Greyhawk Castle Ruins

The fallen west gate of Greyhawk Castle was at hand, and through this mouldering portal the party passed. In a few moments they had entered the great central keep, heaved open an inner door, and carefully proceeded down a set of winding stone steps---steps worn with age and slippery with dampness. They had entered the dungeons." [4]

Castle Bodenburg, that served as inspiration for the Upper Works.

In the original campaign, [...] the first level of Zagyg's dungeon was pretty much where the adventuring began. No great pains were taken to detail the ruins above the dungeons. Sure they existed, but the adventure was chiefly subterranean, and in those formative years of fantasy role-playing, dungeon exploration was what the players most desired to do. [7]

Gary had a model of Bodenburg Castle (he called it "Castle Bodenstadt") in his basement, and used it as the original model for the ruined upper works of Greyhawk Castle, elongating some curtain walls to make it match the full rectangle of graph paper he had for the dungeon. [15]

The castle is not a model of Bodenburg. The name came from the "Siege of Bodenburg" game, that had Rules written by Henry Bodenstedt, and inspired Gary to write the Chainmail Rules.

In the first incarnation, Castle Greyhawk was to have no substantial above-ground presence. A few rats and minor bandit nuisances only, with entrances to the real adventure hidden by rubble, so the "upper works"  received short shrift. Four minor encounter areas, and a pit area where an adventurer of incautious sort might slide down and take damage, but possibly find another hidden entrance to the first level. All done! [15]

The original ruins were an elongated version Bodenburg Castle as Scott points out. This of course only has one central tower, a gatehouse, bartizan, and building surmounted walls (which might have 1 or 2 towers incorporated amidst buildings). [PS]

The Central Staircase

[T]he wide staircase provides access through levels 1 to 3. [7]

Gary briefly described the castle ruins and broken walls and outline, and then we immediately descended down spiral stairs into the dungeon. We get almost at the bottom of the stairs - about to walk into a room, when a large spider drops down from the ceiling and bites the lead PC. He fails his check and dies of poison! [14]

The central staircase went down a good 125-150 feet - at fifty foot intervals were doors. We ignored these and continued to go down. Eventually we came to bottom and a landing that led to another narrow hallway heading in a northeasterly direction that went on for a good 80-100ft, which abruptly ended in a dead-end. [12]

The stairs have been described to go down to level 3, but the purported map of level 3 does not match this play report - there is no dead-end corridor with secret door to the North-East if we assume N is up -- and the round rooms of level 1 and 3 align, so no different compass rose either. Level four is supposed to be undead, and they encounter a wraith. 

Random Encounters

Six robed men who wear pointy hats with symbols on them, low level magic-users who charm person. Old school charm person, good for a long time. From the play reports it sounds as if wizards essentially consisted of sleep (against groups of weaker monsters) and charm person (against strong individuals). 

[References: see Greyhawk References]