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 enough, 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 Laboratories:

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