Watcher's Keep is an ancient castle that was built to be a magical prison holding creatures of the dark. For ages the keep has been observed and guarded by Helmites, followers of the God Helm. A group of Helmites known as the Knights of the Vigil were told by Helm to ensure that the "Imprisoned One," Demogorgon, would never escape the keep, and many knights sacrificed themselves to ward the castle. Eventually, the Knights lost their confidence to tribute themselves to the cause of sealing away the Prince of Demons and spread messages throughout the lands pleading for help.
Watcher's Keep is the only area which you can access in both Shadows of Amn and Throne of Bhaal. Below the Keep's stairway are the Knights of the Vigil, and they will ask your help for strengthening the seals to the Imprisoned One. If you agree you will be taken to the entrance of the first level of Watcher's Keep. You will need to go to Brother Pol to collect the Ritual Scroll and the Vigil Stone, without which you cannot enter/leave the Keep. Sister Garlena has supplies you can buy. On the left side of the keep are teleports to each separate level. Before you can travel through them you will need to have reached that level from the inside. Other than the Knights and the Keep, there is nothing of note in the area.
There are six levels in Watcher's Keep; The Ritual level, Elemental Level, Maze Level, Machine of Lum the Mad, The Final Seal and the final level where the Imprisoned One is. Each of the first five levels has a different way of getting to the next level down.
On the Ritual Level you need to collect the bell, the candle and have made a note about which page of the book you need to read. You then need to place the bell with the candle alongside the book on the altar. Reading the ritual correctly will cause the teleporter to the next level to open.
On the Elemental Level you need to collect the Air, Slime, Fire and Ice scepters and place them in the four slots around the Chromatic Demon. You need to then kill the Demon and collect the key off its body to go down to the next level.
On the Maze Level you will first need to collect the Scepter of Radiance and find your way across the rooms correctly, collecting the three scepter gems along the way. Once you have the three scepter gems and they have been inserted into the Scepter of Radiance, you can go through the portal to the next level.
In the Machine of Lum the Mad level you need to collect the blue, purple and red oil, as well as the flint and tinder in order to gain access to the Crystal Mallet. You can then break the glass on the Machine of Lum the Mad and perform the right actions to activate the portal to the next level.
On the Final Seal level you will need to collect the Mind, Spirit and Heart key from the three challenges on the level and defeat all the guardians of the Final Seal. After all three keys have been inserted into their respective slots, the final portal to the Imprisoned One will appear.
When you are down in the sixth level, the Imprisoned One will talk to you and tell you that the Knights of the Vigil are lying to you and are being deceitful. Whether you believe the demon or not or read the ritual or not is up to you.
For a much more detailed walkthrough of Wathcher's Keep see Watcher's Keep Walkthrough.