In Friuli and Veneto there are a number of fortified towns. Probably the three most impressive Citadella (the name just says it), Montagnana and Palmanova, the latter one is the best in respecto to urban planning (the becauty and symmetry of Palmanova is only by few cities such as Siracusa in Sicily).
These cities offer a unique insight into a long-lost world.
Halfway between Treviso and Vicenza, Citadella is really a in itself closed fortress. Therefore it is of interest for those who wish to study the function of a medieval defense system.
Apart from that this city, as Montagnana, is located in the Veneto and mentioned here for completeness sake.
Located south of Vicenza, Montagnana is the city life of today in middle of an old scenery. Thus it will carry away all those who want to experience the the atmosphere of history.