Dungeon Level

The Dungeon Level was the first level of Undermountain.

Yawning Portal
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).

Gates
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.

Sewers
Waterdeep's sewers and Undermountain had several connections. Criminal organization and guilds were, at least according to rumor, in control of these connections.

A sewer entrance that was under nobody's control led to the Dungeon Level. It opened directly above the Grim Statue, a 60-feet high (18 meters) room with a partially destroyed 40-feet (12 meters) statue. People had to come with their own ways to get to the ground and to deal with an electrical discharge similar to lightning bolt that happened every eight minutes. One way to deal with the latter was to cast dispel magic on either the lightning bolt or the statue. Doing the former dissipated the bolt, doing the latter cancelled the statue's next release. The door automatically closed at latest after a dozen days, something Halaster made sure happened. The statue itself hosted a secret entry into the Arcane Chamber, the second level, in the form of stairs that spiraled below. The statue was located in the south-south-west of the Entry Hall.
 * The Grim Statue:

Notable Locations
This room contained a rare sight within Undermountain, a so-called bottomless pit that was not obscured by a curtain of darkness. This place acted as a way out from Undermountain.
 * Chamber of the Well:

This was an archetypical temple to the drow deity Selvetarm, which was found in Undermountain south-south-west of the Entry Well.
 * Chapel of the Sericeous Sargh:

This throne was found south-west of the Entry Well. In 1371 DR, there were feet-bones at the feet of the throne as though somebody was sitting on it. The rest was floating around on the throne. The skull always faced onlookers. What happened there was that some priest tried to create an undead servant, but the process so hard that it did not just kill him, but made his bones float around. Small objects could be made to join the floating stuff and the entire floating could be made stop by employing dispel magic.
 * Dead Man's Throne:

The Falling Stair was a landmark, a stair whose roof fell when people were within 50 feet (15 meters) making it completely impossible to pass through except by digging through or by waiting for ten to forty minutes at which point the rubble rose back within three minutes. The stair led to the Citadel of the Bloody Hand, if not for the topmost section being walled off by the City Guard of Waterdeep, while the actual stronghold was under their control. Everyone who went up the stairs had to pay the so-called "Lord's take", meaning everything except for 100 gp per person, but received healing.
 * Falling Stair:

The Falling Wall was a wall that was located north of the Chamber of the Well. It was a wall that did not just fall on those nearby, but also pulled those within 3 feet to itself to crush them underneath. Due to illusions, it was impossible to find out that the Falling Wall was not an ordinary wall. Halaster also cleaned up blood stains and so on that might tip off people. While it was possible to delay the fall by one minute with dispel magic, it was not possible to reverse the process, that was something for which a person needed to wait for twenty to fifty minutes.
 * Falling Wall:

A similar structure existed nearby the Chamber of the Well.

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:

This room, or more a corridor, was south of the Entry Well. There were a lot of floating candles, not hundred but sixty of them. The effect was fashioned by Muiral, a mad apprentice of Halaster and the developer of the candle spell. The effect allowed the hot candles to perpetually burn, outside of the corridor, they burnt only for ten minutes or so, and made them impossible to put out. However, casting dispel magic on even one of them put out all of them at once. It was believed to be the only place where Muiral's mind could experience some lucidity.
 * Hall of a Hundred Candles:

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:

Located west of the Entry Well, the 30-feet-by-30-feet (9-meters-by-9-meters) Librarium could only be entered through a secret door to the west. On opening it, an apparition of human lips appeared that told them that people should not speak.
 * Librarium:

The room itself was furnished with bookshelves at the walls of various topics. Halaster put a cloud-like trap at the room's ceiling, which descended on people who ignored the warning and spoke. When they made this mistake twice, the cloud completely descended around the speaker's head, muting the person for ten minutes and sapped one spell from the victim's memories every time speech was attempted.

South of the Entry Well was a giant magical portcullis that shut a corridor. That one fell from the ceiling at some point before 1371 DR. Somebody actually bent the bars in the center to create a gap trough which people could walk through. The aforementioned magical properties of the portcullis was of reflective nature. Every single spell that was cast on or through it was reflected or dispelled.
 * Portcullises:

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.

South-south-west of the Entry Well was a 60-feet (18 meters) high room with a throne guarded by battle horrors. When a person sat on this throne, it started to glow and the person experienced a teleport effect that sent the person away. The exact destination changed over time, but known destinations were: a forest in Cormyr, the demiplane of Dread, namely the domain of Verbrek, another planet, the Rat Hills, the Sea Stacks, the lair of a vampire in the north of Waterdeep, and some hills south of the High Moor.
 * Sundered Throne:

East of this room was another one that hosted one of Halaster's traps, a pillar with gems. It was possible to pluck out gems from this pillars, but when a person grew too greedy and took out seven or more, Halaster's trap sprang in the form of a necrophidius that stole a magic item from that person, for example by biting of the hand holding one, or killed the person in the case no magic item was held by him or her. Once a magic item was seized, the worm went inside the pillar, where the magic item disappeared. The entire trap served two functions for the Mad Mage: first, entertainment. Second, as a source of magic items.

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:

Situated west from the Entry Well, Whitehelm's Tomb was the tomb of Bereg Whitehelm, a dwarf turned spectre of whom and his deeds nobody really knew anything. The undead sought to kill intruders to have additional spectres under its control.
 * Whitehelm's Tomb:

The tomb itself had several rooms and people entered it by falling into it via a trap that consisted of a de-materializing floor-piece. Once in the tomb, the western side had fourteen coffins of Bereg Whitehelm's servants and at the westernmost wall was a secret passage that allowed exit, but not entry except with a knock spell. There was also a corridor to the east that led to a granite statue of a dwarf that expressed a dire warning against intruders via a magic mouth spell when somebody came within 10 feet (3 meters) of the statue. There was also another secret entry and exit point on the eastern side.

The actual tomb could only be entered by somehow bypassing the aforementioned vanishing floor-piece. One entered a 60-feet-high (18 meters) room, went up a dais, opened a secret door, and then one could find the actual tomb where the spectre also lived in 1371 DR.

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.

South of the Entry Well was a continuously working gate that caused people to teleport to the Hall of the Black Helm in the second level of Undermountain.

As mentioned above, the Chamber of the Well served as an exit from Undermountain. However, it required one to follow an odd procedure. The west of this room had an entry from the north and one from the south. To benefit from the teleportation effect, a person needed to enter the room from the northern entry, circle the bottomless one time, go back to the entry where he or she entered from, and face it. Once all this is done, the teleportation occurred and one was teleported to Southfort Keep in the south of Waterdeep. Naturally, a person who followed the procedure by accident was affected by the magical effect too.

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.