I adore touring castles. I've been to Chillon, Gruyeres, and a few on the Rhine. Yes, Carcassone is overflowing with tourists and Rick recommends spending the night. My situation is hubby and I will be arriving from the West Coast to Toulouse on April 28th. Based on previous trips, we expect to be quite jetlagged. We're thinking of 2-3 nights in Toulouse; we need to be in Nice the night of May 4th to start a hiking trip the next day. Carcassone is on the train line towards Nice. Should we plan on 3 nights In Toulouse and do a day trip to Carcassonne, returning to Toulouse late in the evening? Or 2 nights in Toulouse and 1 in Carcassonne? We don't generally like 1 night hotel stays. Still deciding where we should stop en route to Nice. We've been to Nice, Avignon, Nimes, Aix, but not to Marseille, Cassis, Narbonne, Montpelier, all of which have been suggested. Our week-long self-guided hike is in Haute Provence, including the Grand Canyon of the Verdon and other rural places. We like big cities, rural areas, small towns...if it's in France, we'll probably be happy! We're ending the trip in Paris. Thanks for suggestions.
The train ride from Toulouse to Carcassonne is only about 45 minutes, and my recollection is that there are multiple trains each day on that route, so a day trip should be very feasible. We actually did it in reverse: stayed in Carcassonne and day-tripped to Toulouse. But you should check the schedule on SNCF to make sure the timing works for you.
For the record, the old walled city in Carcassonne is indeed very busy in summer, but things move along quite well. And the new city is very nice. We enjoyed the week we spent there.