The Conciergerie was a barracks building for royal troops.  Everything necessary for the upkeep of military men was here:  dormitory, mess hall, stables, armory...  Today it's just a museum showing how the working class lived. 

Being a museum of sorts, it also has a few exhibits.  We were particularly taken by...

...the scale model of Notre Dame because of its attention to detail.

This shows 'the forest', the section above the ceiling that supports the metal roof of the cathedral.  This is the area that burned April 15, 2019 and that could have caused the entire structure to collapse.  Fortunately, the fire was extinguished before that happened.

The beams of the forest were 800-year-old oak beams, so they burned really well.  Replacing them would have been a decades-long project except for a lucky turn of events. 

About 20 years ago, a group of architecture students working on a term project asked for permission to 'map' the forest.  They brought in laser measuring devices and got exact dimensions and positions for all of the wood that made up the structure holding up the roof.  After the fire, it became possible because of that 'map' for teams to prospect among France's oak forests to find mature trees that, one by one, could replace each of the damaged beams.  The result is that 'the forest' has been reconstructed to the form it had when Notre Dame opened to the public in AD 1250.

That's a miracle.