Citadel

The Citadel is the home of Merlin, Gaius, and the royal family. It is the seat of government for the kingdom of Camelot, and is surrounded by the Lower Town.

Arthur's chambers
Arthur's room has been shown in many of the episodes in the Merlin series. This is where Prince Arthur sleeps, eats etc. During the events of Goblin's Gold, Arthur  had a lot of gold coins which the Goblin (trapped in Gaius' body) stole. He had won the money in a competition.

Uther's chambers
Uther's chambers bear a resemblance to Arthur's Chambers. Gwen was reveled to be looking after him during the events of Series 4 (The Darkest Hour). Uther's chambers have appeared in several episodes of first four series but haven't been seen since the death of Uther in The Wicked Day.

Burial vault
A vault beneath Camelot Castle houses a number of sarcophagi, each containing the remains of a deceased noble.

Arthur Pendragon mentions the vault as being on the route along which he helped Mordred escape the Castle (The Beginning of the End).

Among the dead in the burial vault was Tristan de Bois, whose remains were reanimated by Nimueh so that de Bois could take his revenge on Uther Pendragon (Excalibur).

It is almost certain that the remains of Ygraine de Bois reside in this burial vault as well.

Water vault
A vault beneath the Castle of Camelot contained a large storage unit that supplied the city with drinking water. The vault appeared to be accessible only through a single locked door that lead into a downward stone staircase and into an expansive cavern system. It is currently unknown if the water vault connected to the Dragon's cave or to the burial vault that also resided below Camelot.

The year that Merlin arrived in Camelot to begin his apprenticeship with Gaius, the sorceress Nimueh magically transported the egg of an Afanc into Camelot 's water supply, releasing a deadly plague into the city. After Gaius and Merlin  deduced that the pathogen was spread through the water, they descended into the vault and discovered the Afanc. Merlin later returned to the vault with Arthur Pendragon and Morgana, and aided Arthur  in the destruction of the creature (The Mark of Nimueh).

The water in the vault turned to sand after Arthur killed a unicorn (The Labyrinth of Gedref).