It's the most famous church of Paris, and even of the world, with a unique location on the "Ile de la Cité", and its typical Gothic style. Notre Dame de Paris stands on the site of Paris' first Christian church, Saint Etienne basilica, which was itself built on the site of a Roman temple of Jupiter!
The construction of the current cathedral began in 1163, under the reign of Louis VII. Over the construction period, numerous architects worked on the site, as is evidenced by the differing styles at different heights of the west front and towers. The Rose window was done between 1210 and 1220 with the fourth architect, and the cathedral was completed around 1345.
If a bit of exercice doesn't scare you, after about 300 steps, you'll arrive on a gallery joining the South and North towers, and you can admire the famous gargoyles as well as the nice view over Paris. And if you're not tired yet, you can climb an other 115 steps inside the South tower to reach its top!
Free entry for the first part. To go up the towers: 7 euros for adults, free under 18 years old. Entrance Rue du Cloître Notre-Dame, north-west angle of the cathedral. Tel.: 01.53.10.07.00 or 02