The Ragged Flagon Cistern is located through a cupboard in The Ragged Flagon, just before the entrance leading to The Ratway Vaults. After entering through the above area, it can also be accessed through a hidden entry under a small tomb next to the chapel. After using this entrance for the first time, it will be marked on your map, to the immediate right of Riften's marker, as "Thieves Guild".

It is the Thieves Guild headquarters and often where one will find Brynjolf and other Guild members throughout the questline. Upon joining the guild, it's possible to use this location as a player home.

After the Thieves Guild Questline is completed, a Shrine of Nocturnal can be found here.

This is where Mercer Fray and Brynjolf are usually found during the questline.

The Dragonborn can obtain an infinite amount of steel arrows at the archery range when Niruin is practicing, because he wields an infinite amount of steel arrows and shoots at a fast pace.

If you Pickpocket his arrows and replace those with one arrow you want he will shoot those. So if you put daedric arrows in his inventory, he will shoot infinite daedric arrows.

Also, there is a room with a Master, Adept, Expert, and Aprentince chest. These are for practice, and you can lockpick these chests repeatedly to improve your lockpicking skill.