The Conciergerie, on Paris's Ile de la Cité, became notorious as a revolutionary prison: Marie Antoinette and Robespierre were both held here before their executions. However, the building's history goes back far further and while the upper levels are dedicated to this prison history, the ground floor is an impressive piece of mediaeval architecture.
In fact, the ground floor is now some way below modern street level, so you enter by descending a staircase, and the windows are blocked giving it something of a cathedral crypt ambience. It is all that remains of the palace which originally occupied the site. Charles V left this palace for the Louvre after a popular invasion in 1358 which had seen his father's advisors killed, and created the post of Concierge to administer the building and its prison. By the following century it had become one of Paris's largest prisons.
The salle des gens d'armes dates from 1302, and was restored in the nineteenth century. It is the largest secular mediaeval room surviving in Europe, 64 metres long. To warm such a large space, each of the four walls had its own fireplace. Since the palace employed several thousand staff, the generously-sized space was very much needed! Staircases led up to the Great Hall, which burned down in 1618. However, its antechamber, the Salle des Gardes, still survives and features some fascinating column capitals: