I did a bit of scouting myself earlier this year as I was due to visit last month, (had to cancel at the last minute though). Guidebooks don't seem to have much information on the citadel. However one guide (I think it was the Cadogan one) states that it is open to the public to walk around most of the time and that there are guided tours at weekends.
The
official website seems to confirm this, stating there is a Promenade showing the details of the fortifications. . However my French is not too great so I may have missed some of the detail. There also seems to be an exhibition open to have a look around.