The castle (more a fortress) is in good state and is quite big an impressive. You can walk through the inside for free, and to the outer limits of the castle, offering you a great view of Dieppe. Only a small portion of the castle is used as the museum, which on its own is not very interesting.
The free visit is worth it, a) because of the impressive structure and b) the view.
If you own or manage Chateau Musee de Dieppe, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.