This blog really is becoming a large filing cabinet of travel information for me. Conde Nast Traveler has a list of things that are hidden about Notre Dame, kept here for posterity.
And because I'm worried that this link won't last forever here is the list copied here:
Originally, spires were designed for the towers of Notre Dame. It's a mystery why they were not actually built. Money wasn't an issue, as construction of the cathedral continued after the towers were erected. Perhaps it was thought that the building was high enough; even without the spires it towered high above the houses that crowded close to the front.
Huge cloisters used to exist to the left of the front of Notre Dame, where there is now housing, but they were destroyed by fire in the 17th century. It was in these cloisters that the University of Paris began, as a theological school for training clergy. The University was born well before the present cathedral, and also before its predecessor, the Romanesque cathedral of St. Stephen, built around 1100. Nobody is quite sure when the school debuted, but it had to have been during the time of the much earlier Carolingian basilica that stood on the site. It was only in 1253, when Robert de Sorbon, the then-bishop of Paris, had an edifice built to house it, that the school moved indoors and took the name Sorbonne, in honor of the bishop.
St. Anne Portal
The cathedral's right-side doorway is an example of medieval recycling; you can tell because it is not a true Gothic portal. Notice the rounded arch marking what was the top of the door above the head of the Virgin Mary. It was the main portal of the Romanesque cathedral torn down to make way for Notre Dame, but it was reused here by setting it into a Gothic arch. A closer look reveals how stiff and stylized the figures are, compared with those on the other two portals dating from more than 100 years later. Another interesting point is that all the male figures depicted on the tympanum of this portal are wearing pointed hats. Why? Because these men are all Jews, and pointed hats were worn by Jewish men in medieval France.
There are fascinating representations of the Theophilus narrative, depicted twice on the north side of Notre Dame. It is a Marian miracle tale that is basically a medieval Faust legend. A clerk sells his soul to the devil to obtain a higher office, regrets his decision, prays to the Virgin Mary who then goes into hell to obtain his contract. Once she returns it to him, he lives a virtuous life and goes to heaven upon his death. Since the north of Notre Dame faced a cloister with canons living inside, it makes sense that this was an "anti-ambition" message that the men should stay in their place.
The 19th-century restorers who worked on medieval churches and cathedrals loved to leave something of their own time and often of themselves. Viollet-le-Duc, who was in charge of the renovations, was no different. Rising up to the base of the spire, which, incidentally, is Viollet-le-Duc's ideal version of a medieval spire, you'll see four groups of three men in bronze, now green from oxidation. These are the 12 apostles. The top apostle on the south side of the peaked roof of the transept has his elbow in the air, with his hand shielding his eyes. This is St. Thomas, the patron saint of architects. But it's also a self-portrait of Viollet-le-Duc himself, looking up at the spire that he has built, no doubt admiring his own efforts.