Tol Dagor is one of the few dungeons in game where the entirety of the dungeon is available to be explored by the player outside of the dungeon itself. Just as in the dungeon, you can enter the prison through the sewer tunnels, or fly into it from above. It is, however, empty outside of several quests that take you inside.
Tol Dagor shares a number of similarities with the Vault of the Wardens from Legion. Both are prisons which first serve as part of the starting experience, then later dungeons for their respective expansions.
Tol Dagor is named similarly to Tol Barad, which used to be owned by Kul Tiras, which is also why it shares a similar architecture.
Much like Tol Barad, the name "Tol Dagor" is derived from the Quenya and Sindarin languages created by J. R. R. Tolkien for his legendarium. "Tol" is Quenya and Sindarin for "island" while "dagor" is Sindarin for "battle". "Tol Dagor" would thus mean "The Island of the Battle" in Sindarin.
During alpha, the quest [10-50] Out Like Flynn said that the Ashvanes threw anyone in; people they didn't like, people they wanted to keep quiet.
In the Map & Quest Log interface, the Tol Dagor dungeon is shown as part of Tiragarde Sound while the outdoor zone is shown as a separate zone of Kul Tiras. However, the Flight Map also shows the outdoor zone as a subzone of Tiragarde Sound.
At one point during the alpha, there was a Cell Block: Karybis and Cell Block: Scillus. Karybis was part of the Brig and Scillus was renamed to Detention Block.