From Tolkien Gateway
The Twenty-first Hall was a great hall on the Seventh Level in the North End of the Dwarf realm of Moria.
Balin's Colony reclaimed the Hall from the Orcs. There is a mention that there was a "good air" and "the shaft was clear", and allowed the Dwarves to watch their surroundings.
Years later the Fellowship camped in the hall for the night on their trek through Moria. Gandalf was able to identify the hall after the Fellowship had found the Chamber of Mazarbul.
Portrayal in adaptations
2007: The Lord of the Rings Online:
- The Twenty-first Hall is the central base of operations for the Iron Garrison.