This is a single-level branch reached by a magic portal from the Dungeons of Doom that is located in a vault between level 11 and Medusa's Island (and therefore between DL 11 and DL 27). Each vault created within that range has a 1/3 chance of containing the portal if it has not already been generated (except vaults created on the level with the portal to the Quest; note further that no vaults will be created on the Big Room or Rogue levels). As a result, Fort Ludios may be inaccessible in any given game if no vault happens to contain the portal.
The Fort itself has a nominal depth of between DL 18 and DL 21 for the purpose of evaluating monster difficulty and other calculations dungeon depth is used for.
Fort Ludios is ineligible to leave bones files. The entire level is no-teleport.
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Legend
You arrive on (and leave) the level at the magic portal marked in the lower left. All walls on the level are undiggable.
Croesus occupies the throne, with the remainder of the throneroom being filled with the standard throneroom monsters; the zoo and barracks get their regular allocation of random monsters and soldiers and officers respectively. In addition, sixteen soldiers and a lieutenant are scattered around the courtyard surrounding the fort, together with four random dragons; four giant eels occupy the moat.
Each square of the vault contains 600 to 899 zorkmids, and has a 1/3 chance of containing a trap (2/3 land mine, 1/3 spiked pit). The four corner towers contain three diamonds (NW), three rubies (NE), three emeralds (SW) and three amethysts (SE).