The Palazzaccio di Marcignano is the name by which today the ruins of the castle of the Da Gavignano are identified. The fortress formed a defensive triad with those of Tizzano and Musignano, located a little way from the Maremmana road, protected and connected (legend has it with a mysterious underground passage) to the outpost consisting of the overlying Gavignano building. The structure, of considerable size, is in a poor state of conservation and completely covered by vegetation, in particular from ivy. Only the perimeter of the tower can be clearly recognized. A fault, easily detectable today on the ground, must have been the first cause of its abandonment due to the unstoppable disruption that allowed to remain standing only to the aforementioned tower, reinforced by bastion walls joined to a high wall embellished with an elaborate redundant and elegant windows, among which stands out what remains of a mullioned window decorated in sandstone, subsequently buffered. Some architectural elements with a military function are still identifiable, such as slots for crossbow shooting. It was listed as a tower with fortress and in good condition in 1583, according to the drawing on the Plants of Peoples and Roads (1580-1595), in which the crenellated perimeter also appeared. The interior spaces, whose floor plan was much lower than the current one and had to be covered by brick cross vaults of which only the shutters of the arches remain, is datable to the first four hundred, although it is almost totally invaded by the large chestnut trees that they grew up after the abandonment. We find a well, perhaps connected to a cistern, and we can see remains of rooms and other structures.