Dungeon Level

The Dungeon Level was the first level of Undermountain.

Entrance
The dungeon of the Undermountain was accessible via a winch over the the pit in the common room of the Yawning Portal, with a fee of 1 gp per person. It took roughly a minute to lower or raise the rope-and-pulley mechanism that powered the machine. The well descended for 140' (42.7 m) to a floor of rough-hewn rock that was covered by a thin later of sand and the skeletal remains of a number of different creatures, some of which were humanoid in form.

Sixty battered and cracked shields hung on the walls of the entrance chamber. They featured the heraldry of many Waterdhavian familial crests, some of which were recognizable as belong to the noble families of the 14 and 15 centuries DR. These ancient shields were so fragile that they broke when touched. Behind one of the shields was a message written in blood in Elvish:

"Beyond the pillar forest, the Mad Mage waits. Casting spells behind magic gates."

The "gates" likely referenced the gates required to transport between the levels (although other passages could also be used for that purpose).

the Dungeon Level held at least one portal that connected it to Calimport, which was used by an adventurer company called the Black Banners in 1371 DR that met its end at the hand of a group of bugbears.

Notable Locations
By 1371 DR, the white-tiled room decorated with mosaic that held the Fountain of Flowers at its center was in ruin. The actual fountain was made of granite and looked like a pillar made of flowers from which no water came out, because the pipes did not work anymore. At the ground of this fountain was a passage that could be opened with the proper key or through the skills of a competent rogue. The passage led to a destroyed temple of Bhaal. The culprits of its destruction were the Lords of Waterdeep.
 * Fountain of Flowers:

A long hallway leading from the entrance chamber opened up to this 20' (6.1 m) long hall, which featured a series of bas-relief sculptures set into the walls, 4' (1.2 m) wide, 9' (2.7 m) tall and 3" (7.6 cm) deep. Each wall depicted different species of demons, including a balor, a marilith and a shadow demon, among others.
 * Hall of many Pillars:

A hidden tunnel could be accessed behind the relief of a nalfeshnee on the southern wall, which led south slightly downwards towards a small chamber with a westward-slanted floor, that had been partially flooded by the fetid sewer waters of Waterdeep. A 20' (6.1 m) wide alcove set into the north-facing wall held a life-size statue of a sahuagin, half-submerged in water, shining with a purple glow. The head of the statue could be unscrewed at the neck.

On the northern wall, behind the relief of a dretch, was a small hidden room that contained an unadorned, wooden armor stand. Anyone who entered this chamber heard the echoes of far-away voices that echoed through 10" (25.4 cm) long vents they were set into the ceiling.

The western portion of the hall opened up into slightly wider, larger hexagonal room that housed a number of smooth-stone pillars. On the southwestern wall someone had carved the words: "CERTAIN DEATH, THIS WAY!" in thorass. A large, serpentine skeleton was entwined around the northern-most pillar. The entire room was considered a dead-magic zone.

West of the pillared room was a 100' (30.5 m) long hallway, that featured 16 heavy oval-shaped mirrors, divided evenly among the northern and southern walls. These mirrors were enchanted with magic designed to preserve their silver backing from the room's moisture. Three of these mirrors hung before gaps in which objects could be hidden in. These gaps changed from time to time places and could be behind different mirrors. These mirrors could be destroyed as easily as any other mirror, but destroyed ones were replaced with while ones. One of these mirrors, to be precise the one in the far west, was a mirror of opposition that created an aggressive double that tried to kill first the reflected person and then everybody else who was present.
 * Hall of Mirrors:

These were two stairs in a mist-filled corridor, one in the west, the other in the east, but were neither visible or accessible to each other. The western one consisted actually of two stairs that rose, reached a landing, and then descended again. The eastern side of this pair of stairs had the peculiarity that one of its steps swiftly rose a feet and then descended again, thus tripping people. The landing sported an apparition of an elven warrior that was not dangerous at all, people could simply go through the image. This image did not detect as magical, but could be easily dispelled to disappear for up to forty minutes. The landing also had a magical bracket set by Halaster. By putting a burning torch inside it, the torch started to burn brighter, without using fuel, and dispersed the mist that covered the mist of unknown origin.
 * Sinister Stairs:

The second stair was one that descended to the Black Boudoir in the second level of Undermountain, although the last part consisted of a teleport effect. Halaster set a trap in this place. Namely, the wall, ground, and ceiling started to set themselves on fire, which was naturally harmful to those within the area. It was possible to alleviate the harm by flying instead of walking on the fire, albeit the movement was slowed due to the updraft. While it was possible to suppress the fire by means of dispel magic for two to five minutes. Halaster prepared for this eventuality, when aforementioned happened, an aggressive salamander appeared that was too fearful of the Mad Mage to have a meaningful conversation with others.

This temple was accessible via a teleportation device set by Halaster that was specifically designed to circumvent his ban on such magic.
 * Temple of Gulkulath:

Exits
Aside from the winch system (see above) going to the Yawning Portal, there was a staircase in the south corner of the dungeon that led to Level 2, the Arcane Chambers. In a room in the south-west part of the dungeon was a magic gate to Level 10, Muiral's Gauntlet. The following conditions applied to it:
 * The gate opened for 1 minute when the mirror was touched with a magic wand that had at least 1 charge remaining.
 * A would-be traveler must of sufficient experience.
 * The first creature to pass through the gate triggered an elder rune.
 * A creature that passed through the gate appeaeds in a room in the west part of Level 10, Murial's Gauntlet.

15 century DR

 * Undertakers:
 * In 1493 DR, this gang of bandits preyed on gullible adventurers, demanding a toll of 10 gp per person for safe passage on this level. The bandits were failed actors and singers who used disguise kits to appear as vampires. One of the leaders, Harria Valashtar, had a flesh golem.


 * The Xanathar Guild:
 * Nine bugbears (five under the control of intellect devourers) represented the Xanathar Guild on this level. They were supposed to keep adventurers from reaching lower levels.