The Fortress, which occupies one of the bastions with fortifications also in its inner part overlooking the city centre, contains the main part of the ruins of the medieval walls: the Cassero and the gate Porta di Santa Lucia. Above the entrance gate of the fortress is the coat of arms of the Medici, with an inscription in memory of Ferdinando I: the construction was completed under his rule in 1593. The interior features a graded ramp, which leads to the parade ground. There you can find a well with the coat of arms of the Medici and a cistern for the collection of water. The Cassero differs significantly from the other buildings, both for the building technique, and for its vertical thrust, which was a typical feature of defence walls prior to the introduction of gunpowder. Originally, the Cassero consisted in a gateway with its preceding defensive gateway and an imposing tower. The shape of the Cassero was significantly altered by the use of the Medici: the medieval buildings were partly buried, the tower lowered and the gate closed and covered. The exterior part of the gate arch features the coat of arms of Siena and the inscription with the construction date: 1345.