The cathedral of Notre Dame de Paris is one of Paris' most famous landmarks. Situated on the Île de la Cité, in the middle of the Seine, it's an unmissable landmark. This Roman Catholic cathedral began to be built in 1163, making it over 850 years old. At the time it was one of the first French Gothic buildings to be built, and it still stands as a magnificent tribute to the style, and one of the largest churches in the world.
There are so many different attributes to the cathedral that it is impossible to describe them all, but it is notable for its organ, stained glass, figures, façades, paintings, sculptures, treasures, and even its furniture. While many people will tell you 'it's not all that special, it's too crowded and there are better places to visit in Paris', I would strongly recommend going, whether or not you are religious, for the splendour and space.
Made even more famous by Victor Hugo's 1831 Gothic novel (and subsequent films) The Hunchback of Notre Dame, the bells are noteworthy in their own right, each having its own name.
The front (or West) façade strikes you simply but boldly as you approach, with its clear, strong vertical and horizontal lines, and its twin towers.
The building is as fascinating historically as it is architecturally. Desecrated during the French Revolution (which began in 1789), much of the restoration has not taken place until the 1990s. There has been a series of organs, but now Notre Dame boasts the only organ in France with five fully independent reed stops en chamade, as well as full computerisation. The organ has hosted many famous players, including Pierre Cochereau, whose improvisation skills are extraordinary (a particularly fine example of which are the Christmas improvisations done on the Notre-Dame organ).
Inside there are so many services that the place is likely to be redolent with incense, overwhelming your sense of smell as well as sound and sight.
You can explore the main cathedral, but also climb the towers and enter the crypt. The entrances for these latter two are outside the main building and you should follow separate signs. The Tower and Treasury have entrance fees, but otherwise entrance is free.
If you go to Mass on a Sunday morning, you should be aware that there are two queues outside, one for general tourists and one for those wishing to worship. Make sure you join the right queue so that you make it to Mass on time. It fills up quickly, so it is worth getting there 10-15 minutes early in order to get a seat.
They offer a range of Masses. Two particularly noteworthy Sunday options are the international and broadcast ones. At the international Mass (11:30), readings are in different European languages (English, German and Italian) and translations (including into Spanish) are available in the service sheet, which helpful ushers will give you when you go in. At 18:30 Mass is broadcast live on KTO-Catholic Television and Radio Notre-Dame, making your worship part of a much larger event.
In November 2013 there was also a bank of seat built up outside the cathedral, relaying what is going on inside. On a sunny day this could be quite a good way to get past the crowds inside.
It is very easy to get to the cathedral, even given its apparent isolation on the island. The truism that nowhere in central Paris is more than 500m from a metro station also holds here, and there are lots of bus, metro and train options. You can find out more on the cathedral's website.