The Notre Dame is, without a doubt, the most popular attraction on the Ile de la Cité. Its first stone was laid in 1163, it was finished 170 years later. The Cathedral is a magnificent example of gothic architecture. It is possible to climb the towers and get a nice view over Paris and look at all the details of the sculptures here. It is a bit of a climb, so be reasonably good shape and allow a couple of hours. It is also possible to visit the Crypte Archeologique under the Cathedral, where you can see the foundations of buildings built 2,000 years ago by the Parisii, the Celtic tribe after which Paris is named.
However, Notre Dame is not the only site of interest on the Ile de la Cite. Just a few steps away, you will find another Gothic masterpiece, La Sainte Chapelle built in 1248 by Louis IX, Saint Louis. The upper chamber is a magnificent, stained glass lined vaulted chamber.

Just around the corner from La Sainte Chapelle is the Conciergerie, a Gothic palace that later became a prison. During the Revolution, it housed over 4,000 prisoners, including Marie Antoinette, the latter french Queen. Other points of interest on the Ile de la Cite include the Pont Neuf, the oldest bridge in Paris, that connects the island to both the right and left banks. The park at the western tip of the Ile is a lovely place to watch the Seine flow by. And the Marché aux Fleurs (the Flower Market) is one of the few remaining in Paris.