score:4
I must admit that there is a lack of information about the possibility to visit the cittadella.
After some search on the Internet I came out with a site about Italy's historic monuments (again, only in Italian, sorry) stating that the cittadella is visitable from Monday to Sunday. Moreover, in the description it is said that the place has been open to visitors since 1990. This point is also supported by the fact that in the cittadella there is a museum of the weapons and uniforms of the Italian army.