Dungeon Level

Dungeon Level is the first level of Undermountain. It is accessible via the the pit in the common room of the Yawning Portal, with a fee of 1 gp per person. It takes roughly a minute to lower/raise the rope and pulley mechanism that powered the machine. This leads to a dark room with a tunnel leading south before bending west, a secret door. A thin layer of sand covered the floor, and 60 shields hang on the walls - so fragile that they break if touched. Behind one of the shields is a message written in blood, in Elvish:

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

The gates likely reference the gates required to transport between the levels (although not only they can be used for that purpose). There is a room on the Dungeon Level which contains many pillars, but does hold Halaster Blackcloak - rather, two bugbears with intellect devourers in their skull cavities. A giant constrictor skeleton lies on the floor. The room also holds a secret compartment in the southernmost pillar, which is empty.

Dwellers
The Undertakers

This gang of neutral evil bandits preys on gullible adventurers, demanding a toll of 10 gp per character for safe passage on this level (arguably). The bandits are failed actors and singers who use disguise kits to appear as vampires. One of the leaders - Harria Valashtar - has a flesh golem.

Xanathar Guild

Nine bugbears (five under the control of intellect devourers) represent the Xanathar Guild on this level. They're supposed to keep adventurers from reaching lower levels.

Leaving the Dungeon Level
Aside from the system (see above) going to the Yawning Portal, there is a staircase in the south corner of the dungeon that leads to Level 2 - the Arcane Chambers. In a room in the south-west part of the dungeon is a magic gate to Level 10 - Muiral's Gauntlet. The following conditions apply to it:
 * The gate opens for 1 minute when the mirror is touched with a magic wand that has at least 1 charge remaining.
 * Anyone must be at least Level 11 to pass through, due to Jhesiyra Kestellharp.
 * The first creature to pass through the gate triggers an elder rune.
 * A creature that passes through the gate appears in a room in the west part of Level 10 - Murial's Gauntlet.