Watcher's Keep

Watcher's Keep was an ancient keep of unknown origin, that became associated with the deity Helm and the Knights of the Vigil.

Location
It was located in the realm of Tethyr, west of the Wealdath near the coast.

Structure
The stone keep was built out of a rocky tor jutting out of the ground. It comprised six levels, in descending order from the uppermost floor.

The rooftop entrance to the keep was sealed by a hemi-spherical passageway. It could only be opened with the use of a unique wardstone. Along one side of the keep was a series of doorways that could only be opened from within the keep.

Interior
Each level of Watcher's Keep held a keyed portal that granted entrance to the next level.

The topmost level of Watcher's Keep served as a sort of temple to Helm. It held the private quarters of the keep's priests, a small Helmite tomb, as well as an extensive library that encircled a large central hallway. The hall was decorated with stone statues of various humanoid figures, built alongside four rows of pillared archways, each of which was adorned with a banner that depicted Helm's watchful eye. Within the center of the hall was the floor's portal; at its rear was the altar used by the members of the Vigil, set in front of an enormous statue of Helm.
 * Altar Level:

In order to activate the level's portal, the Helmite prayer ritual had to be performed at the altar in the correct manner. Doing so would activate the statues located in the central hallway, causing them to come alive with great hostility. One of these humanoid statues was equipped with the personal weapon of the legendary Kara-Turan Yakuza named Usuno, while another wielded Foebane, one of the Baneblades of Demron.

The library held a number of notable tomes including the third appendix of Elminster's Ecologies, a golem manual, and fittingly, the History of the Imprisoned One.

As of the mid–14 century DR, this level was inhabited both by the ghost of Giltham the Archivist, as well as more malevolent spirits, such as vampiric wraiths and wandering horrors.

The second level of the keep comprised a central, circular room that was encircled by eight other, a library and laboratory designated for each of the four elements. The central room itself housed a magically enhanced cage and two portals, one that led to an exit outside the keep, and another connected to the keep's third level.
 * Elemental Level:

Within the center room was a magically-enchanted cage created by four archmages, each of whom experimented and studied one of the four elements. They were rivals with one another, often acting against the mage of their opposing element. Trapped within the cage was a chromatic demon, who had been tricked and imprisoned around a century before the Era of Upheaval, in order to secure the Imprisoned One. It could only be freed by using the four scepter keys in the possession of the archmages.

Each of the archmage's respective laboratories and libraries were quite dangerous. The air laboratory had a giant fan that created guardians of air when it was powered up; the slime chamber was filled with toxic mist, that formed as semi-sentient creatures; the air in the ice lab was extremely cold, to where it could freeze a creature solid; and the fire chamber contained a bubbling pool of lava, and granted its fire giant guardian a form of invulnerability. After the archmages were no longer present, the level became overrun with their creations; among these creatures were air elementals, mutated spiders, a giant snake, ice golems, and fire elementals.

The chromatic demon was in possession of the portal key that granted access to the third level of the keep.

The third level of Watcher's Keep comprised a dozen separate rooms, each connected to one another by portals. The rooms were oriented in such a way that portals were located at each of the cardinal directions, or along the north-south axis. To navigate the maze, an individual had to enter through the correct portals in a specific order – with the catch being that entering through a portal in which you already came, did not return you to your previous location.
 * Maze Level:

As of the mid–14 century DR, the level was infested with two groups of fiends who had been transported from their respective homes in the lower planes by the misuse of a certain machine deeper within the keep. Tahazzar led the group of devils, including several cornugons and a pit fiend from the Nine Hells, while the Tanar'ri Ka'rashur led the glabrezu, succubi, and the balor that came from the Abyss. The two fiendish factions continued their Blood War battles while trapped within this labyrinthine domain on the Prime Material plane.

A single crazed adventurer who referred to himself as Yakman managed to navigate the maze level and use several landmarks to map out a route to the level's exit. He even managed to find the scepter of radiance that opened the portal to the fourth level of the keep, though he had lost a few gems to the warring fiends.

The predominant feature of the fourth level of Watcher's Keep was a device known as the Machine of Lum the Mad, an icosahedral device that featured a number of mechanical, steam-powered arms. The operator of the machine was granted access its wide-range of powers, the scope of which was likely not fully known. By manipulating a series of colored dials, shaped buttons and levers of varying lengths, an individual could create a wide variety of effects, such as spawning beings from other planes of existence, conjuring fireballss and bolts of lightning, trapping the operator deep beneath the earth, or even increasing their mental acuity or physical strength. There were a total of 729 combinations that could be made with the various controls, though some of these gave duplicate effects.
 * Machine Level:

Description
It bore its age very well, and all in all, the keep was still in good shape by the mid–14 century DR.

History
The keep was built long before the 14th century DR as a prison for the most dangerous and evil creatures, which the gods themselves wanted imprisoned. For centuries, it was watched by a mysterious group of Helmites, the Knights of the Vigil, ordered by Helm to never let the Imprisoned One escape, or be killed, because doing so would send him back to the Abyss and freedom in his home plane.

To keep the wards holding the Imprisoned One intact, the leader of the Knights of the Vigil regularly sacrificed one of their own. The wards kept the Imprisoned One inside the lowest level of the prison, but the creature's power allowed him to weaken the wards, thus making the sacrifices necessary.

Eventually, however, the Knights of the Vigil lost their determination to sacrifice themselves, and began to spread rumors throughout the land to lure adventurers to the keep, promising rewards for helping them.

Gorion's Ward and companions came to the Keep and had to fight through five levels of the dungeon and withstand its traps and inhabitants. In the end, they met the Imprisoned One and had to decide whether to use a scroll given to them by the Knights to strengthen the wards and keep the demon imprisoned, and risk imprisonment themselves; or choose to fight and kill the demon, which would however free him from his imprisonment.

If they read the scroll, they were trapped, but the Imprisoned One, posing as an innocent kept prisoner by enemies of Helm, could help them escape. Once outside, the Knights of the Vigil ran to see what had happened to the prisoner, then one of their earlier members who sacrificed herself appeared and gave the Gorion's Ward a scroll from Helm, which would trap both the fallen knights and the Imprisoned One.

Inhabitants
The Imprisoned One was Demogorgon, the demon prince and ruler of 88th layer of the Abyss.

Reference
Wachende Festung