This stunning building is called La Conciergerie, a building that used to be part of the Palais de la Cité where the French kings used to live. It is also famous for having been a prison, especially during the French revolution; it's were Marie-Antoinette was kept prisoner until they decided to execute her (it's even possible to visit her cell and many other things if you wish - not before October 8 though). I thought it was ironic that because of the fence they installed to protect Paris Plage, this former prison was currently "caged". It also makes a cool photo ;-)