The double fortress dominating Volterra from its highest point — the cylindrical Rocca Antica (1342) and the rectangular Rocca Nuova (1474, built by Lorenzo de' Medici after the brutal sack of the city by Florence) — together form one of the most imposing pieces of military architecture in Tuscany. The Rocca Nuova's four corner towers and tall central donjon, called Il Maschio, are visible from miles around.
The fortress has been a prison continuously since the Renaissance and is still in use today, which means the interior is closed to the public. The exterior walk around the perimeter is worth doing, especially at sunset when the walls glow honey-coloured.