This is the lowest level of the Dungeons of Doom, located between the 25th and 29th levels.
The Castle is ineligible to leave bones files. The entire level is no-teleport, and is considered a graveyard level for the purpose of undead monsters leaving corpses.
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Legend
This central section is flanked on both sides by narrow sections of maze; the upstair is in the left-hand maze, and the one-way upstair from the Valley of the Dead leads to a random location in the right-hand maze. The only way down to the Valley is through one of the five marked trapdoors in the Castle's back corridor; if these are blocked, the only access to Gehennom is via level teleport (the floor of the Castle level is undiggable through).
There are two entrances to the Castle proper. The drawbridge may be opened either by playing the correct tune on a musical instrument (either by listening to the "Mastermind"-like clues*, or by being given the tune by your god); or by more direct means. The alternative is to use whatever means of crossing water you have available to go round to the back door, though the trapdoors in the back corridor will still need to be negotiated.
The Castle moat contains four sharks and four giant eels. The two marked barracks contain their usual complement of soldiers and officers; in addition, each corner tower contains two soldiers, and the atrium eight soldiers and a lieutenant. The throneroom contains 27 monsters, roughly equally chosen from EHLMNORTXZ; the chest is behind the throne. The alcoves between the marked storerooms contain two random dragons each; the storerooms themselves are filled with weapons, armour, food and gems (each of these in one storeroom, at random). Finally, one of the corner towers at random contains a chest holding a wand of wishing.
*Any instrument capable of carrying a tune (bugles, flutes, harps, and horns other than horns of plenty), when played adjacent to either the portcullis square or the moat square where the drawbridge comes down, can open or close the drawbridge by playing the passtune (answer "no" when asked about improvisation). The passtune consists of five notes between A-G; incorrect guesses will give responses of "tumblers clicking" for correct notes in the wrong position, or "gears turning" for correct notes in the right position. Janis Papanagnou has written an automated solver for this version of Mastermind.