Notre Dame de Paris

Notre Dame de Paris is one of the world's most recognizable cathedrals. It is considered to be one of the finest examples of French Gothic architecture. Notre Dame is also considered one of the earliest cathedral's to use flying buttresses, although these were not in the original plans.
The cathedral is quite beautiful inside, with lots of stained glass and statues. Within it's reliquary several relics are of note, including the purported Crown of Thorns, a fragment of the True Cross, and one of the Holy Nails.
This cathedral was desecrated during the French Revolution in 1790, however it was later restored and has been preserved nearly intact since. There was some damage to the stained glass windows during WWII, and the original windows containing Biblical scenes were replaced with windows with geometric patterns.
There always seemed to be crowds and long lines at this cathedral, but, luckily, the lines moved quickly and we were able to walk through the inside.

The flying buttresses and part of the beautiful garden behind the cathedral.

Oh, I almost forgot to mention, Mary, Queen of Scots was married here in 1558 to Dauphine Francis, later Francis II of France. Mary was 15 at the time. 

Francis, who had been sickly his entire life, died 2 1/2 years later.