We wandered into this quadrangle without knowing what it was. There was a medieval gate, propped open and people were clearly going through it, so we did, too. Down a little medieval alley and into this quadrangle. Clearly it was something incredible, but we only found out later what it was. It was first a cemetary for plague victims in the 14th century, when 3/4 of the citizens of Rouen died. When the plague struck again, they built around it on 3 sides to form an ossuary on the upper floor. Apparently, after being buried for a while, the bones were removed from the cemetary to make room for new dead. The lower level was not enclosed - it was like a cloister. Later, the lower level was enclosed and the fourth side built.
The strange decorations refer, of course, to the cemetary and ossuary and the stone columns supporting the upper level illustrate the Danse Macabre. It's a little difficult to get information about it - most of the information online is in French as if it's a well known tourist site to the French, but not to the rest of the world. The sightseers there the day we were there seemed to be French.
It's now the headquarters of the School of Beaux Arts. The word 'Aître' is from the Roman atrium.