As others have said you could spend the whole day at the Citadel. There are a great many things to see and it is definitely worth a visit. I'd only add a warning for those considering viewing the Resistance Museum. I recall it being all in French with some German. The exhibition is of a desperate and violent period. I felt very lucky it all happened before I was born. It has graphic images of such things as hangings, executions and dead bodies. Once you are in there it is not easily cut short. You are on a threaded pathway. You either go through to the end or turn around to get out. By the time I was half way through I'd had enough and was looking for some fresh air. If you are interested in the period, read French and don't mind the images you could spend hours in there.
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