La Citadelle is a really nice place to visit. You can easily spend the whole day there, expecially if you have children.It is situated up on a hill and there are only 90 parking space available right in front of it. It is better to park the car at the lower parking (space for 400 cars) and take there bus n. 17, which will bring you directly in front of the citadelle. It is a very frequent bus, the ticket costs 1.30 euro and lasts one hour (not enough to see the citadelle and drive back though!).
The entrance to the park outside the citadelle (but inside its first wall) is free and offers a good space where to pic-nic. The entrance to the citadelle itsself costs 9.20 euro, but it is worth it.
There is a restaurant and a sandwich shop inside the citadelle. If you're tight on schedule, avoid the restaurant, since the service is very slow.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.