La Conciergerie is a former royal palace and prison in Paris, located on the west of the Île de la Cité (literally island of the city). It is a part of a larger complex known as the Palais de Justice, which is still used for judicial purposes. Hundreds of prisoners during the French Revolution were taken from La Conciergerie to be executed on the guillotine at a number of locations around Paris.
The west part of the island was originally the site of a Merovingian palace, and was initially known as Le Palais de la Cité. From the 10th to the 14th centuries it was the seat of the medieval Kings of France. Under Louis IX (Saint Louis) (1214–1270) and Philip IV (Philip the Fair) (1284–1314) the Merovingian palace was extended and more heavily fortified.
Louis IX added the Sainte-Chapelle  and associated galleries, while Philippe IV created the towered facade  on the river side and a large hall. Both are excellent examples of  French religious and secular architecture of the period. The Sainte-Chapelle, built in the French royal style, was erected to house the crown of thorns  brought back from the crusades, and to serve as a royal chapel. The  "Grand Salle" (Great Hall) was one of the largest in Europe, and its  lower story, known as "La Salle des Gens d'Armes" (The Hall of the  Soldiers) survives: 64m long, 27.5m wide and 8.5m high. It was used as a  dining-room for the 2,000 staff members who worked in the palace. It  was heated with four large fireplaces and lit by many windows, now  blocked up. It was also used for royal banquets and judicial  proceedings. 
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In 1391 part of the building was converted for use as a prison,  and took its name from the ruling office. Its prisoners were a mixture  of common criminals and political prisoners. In common with other  prisons of the time, the treatment of prisoners was very dependent on  their wealth, status and connections. The very wealthy or influential  usually got their own cells with a bed, desk and materials for reading  and writing. Less well-off prisoners could afford to pay for simply  furnished cells called pistoles, which would be equipped with a rough bed and perhaps a table. The poorest, known as the pailleux from the hay (paille) that they slept on, would be confined to dark, damp, vermin-infested cells called oubliettes (literally "forgotten places"). In keeping with the name, they were left to die in conditions that were ideal for the plague and other infectious diseases which were rife in the unsanitary conditions of the prison.
Three towers survive from the medieval Conciergerie: the Caesar  Tower, named in honor of the Roman Emperors; the Silver Tower, so named  for its (alleged) use as the store for the royal treasure; and the  Bonbec ("good beak") Tower, which obtained its name from the torture  chamber that it housed, in which victims were encouraged to "sing". The  building was extended under later kings with France's first public clock  being installed around 1370. The current clock dates from 1535. The concierge or keeper of the royal palace, gave the place its eventual name.
 Famous prisoners include Queen Marie Antoinette, the poet André Chénier, Charlotte Corday, Madame Élisabeth, Madame du Barry and the 21 Girondins purged at the beginning of the Terror. 
The Terror (5 September 1793 – 28 July 1794), was a period of violence that occurred after the onset of the French Revolution, incited by conflict between rival political factions, the Girondins and the Jacobins,  and marked by mass executions of "enemies of the revolution". The death  toll ranged in the tens of thousands, with 16,594 executed by guillotine (2,639 in Paris), and another 25,000 in summary executions across France.
After the Restoration of the Bourbons ( after the fall of Napoleon in 1814) , the Conciergerie continued to be used as a prison for high-value prisoners, most notably the future Napoleon III.  Marie Antoinette's cell was converted into a chapel dedicated to her  memory. The Conciergerie and Palais de Justice underwent major  rebuilding in the mid-19th century, totally altering their external  appearance. While the building looks like a brooding medieval fortress,  this appearance actually only dates from about 1858. The Conciergerie was decommissioned in 1914 and was opened to the  public as a national historical monument. It is today a popular tourist  attraction, although only a relatively small part of the building is  open to public access; much of it is still used for the Paris law  courts.
Text Source : wikipedia.org