Fort Ludios
Fort Ludios is a special level as well as a branch reachable through a magic portal found in a vault, somewhere above dungeon level 10 and below Medusa. Any vault generated on one of these levels has a 1/3 chance of containing the portal to Ludios, if it has not already been generated. Note that vaults are not generated on the Rogue level or in the Big room, and a vault on the Quest portal level will never contain a portal to Ludios. In some games, the portal is not generated and it is impossible to reach Fort Ludios. It is accessible in 74.9% of all games.[1][2]
The level is ineligible to leave bones files and is no-teleport.
Map
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You enter through the magic portal at the bottom-left of the map. The zoo (with inhabitants marked "z"), fort, and barracks are lit; the other areas are unlit. All walls and floors are undiggable, but all walls and the solid rock at the outside of the level are phaseable. Teleportation is not permitted.
The barracks and zoo have typical complements of monsters and treasures for those room types, as does the central throne room (with inhabitants marked "t"), which also contains Croesus himself, sitting atop the opulent throne. Additionally, 16 soldiers and one lieutenant are in the courtyard around the fort, four dragons are around the moat, and four giant eels are in the moat.
The four corner rooms of the fort contain precious gems: rubies, diamonds, emeralds, and amethysts. The left half of the fort is filled with gold (600-899 zorkmids per square), but also with land mines and spiked pits (1/3 chance of a trap per square, which are 2/3 landmines, 1/3 spiked pits). Levitation will make you able to locate the traps safely, otherwise it is recommended to search all squares prior to moving in this area.
Strategy
Proceeding into the level will find you battling enormous hordes of enemies, the main contingent of which is soldier-types. Many of these enemies are armed with wands, and there are four dragons on the level. Therefore, it is not a good idea to attempt Fort Ludios without reflection AND magic resistance.
If you have a strong pet, such as a vampire lord, you may choose to whistle it across walls to have it fight soldiers without exposing yourself to them. Since monsters will only use their ranged attacks against you, all of their wands will still be charged when you get them. Remember to check on your pet's health periodically. You may still want reflection against the dragons.
There should be dozens of dropped polearms which you can use to kill the giant eels in the moat.
The fort has no drawbridge. To enter, you will need some means of crossing water. If your only means of crossing water is jumping, you can blast through the door from afar with force bolt, the spell dig, or their wand counterparts (wand of striking and wand of digging). If you detect the door with a wand of secret door detection or the detect unseen spell, one of the inhabitants might open it for you. Reading a scroll of earth is also an option - it will create boulders you can use to create a bridge across the moat and/or trap the giant eels.
See Movement tactics#Doors for a way to beat the crowd.
Croesus hits hard, but is easily dispatched with ranged attacks from across the moat. Alternatively, use a wand of cold to form a bridge across the moat; then when he stands on it use a wand of fire to melt it and drown him. Since there are often tough monsters, such as Olog-hai and dragons, in the throne room with Croesus a ring of conflict will help a lot. A scroll of stinking cloud is also quite useful from afar.
Come with a bag of holding if you plan on leaving with all of the gold: Fort Ludios contains around 50,000 pieces, or about half of a strong character's total carrying capacity.
