The walled city of Carcassonne is perhaps the most visited tourist site in southwest France and it's easy to see why. If anywhere looks like it was just ripped out of medieval times and dropped into modern day France, this is the place.
From a distance, the city looks just like what many people would picture as a fairy tale palace and within the walls, there are still quiet sections of the town where you can almost feel as though you were walking along in medieval times even in the peak of the tourist season. Naturally it's very crowded in the main holiday season but because it's so large, you never feel that there are too many people around and as you explore the streets you'll find many parts where you can pretty much be by yourself.
Since it's such a major magnet for tourists, you'll find a range of attractions from the extemely tacky to the exquisite with everything in between. It's even possible to stay within the walls in accommodation ranging from quite small B&Bs to the very elegant (and expensive) Hotel de la Cité (around $300/night).
In Summer you can take the little tourist train around the outside of the ramparts which is thoroughly recommended to give you an appreciate of the scale of the place. After you've taken this trip, the place to go is the Museum of the Middle Ages which will give you a good appreciate of what you will see elsewhere in the walled town of Carcassonne.
Don't miss the tour of the castle and ramparts (the castle is within the walled city) nor the Basilica of Saint-Nazaire (the cathedral) which dates from the 11th century and is still in use today.
Although Carcassonne is by far the greatest attraction in the area there's lots more to see within as little as 30 minutes drive including the Canal du Midi, Lastours (a lovely village with a unique four tower fortress), the Parc Australien, Alet les Bains (where Nostradamus was born) and many others.
The nearest airport is Carcassonne (CCF) served by Ryanair but Toulouse (TLS) isn't far and is served by major and discount airlines giving you a lot more choice in flights.
The city is very well connected by rail and indeed canal.
