The majestic Chigi Palace, built on the hilltop to guard the access roads to the town, dominates the houses that huddle at its feet. It has a 15th century aspect, but it is easy to spot the different phases of its history.
The first building probably dated to the Middle Ages; the existence of the keep was first noted in the late 11th century.
It was remodelled in 17th century when the territory of Formello was sold by the Orsini family to the Colonnas.
The assymetrical entrance holds a rusticated portal in pepperino, now somewhat detioriated. The entrance leads into a covered room and from here, through a large space with a low, rounded vault, into a courtyard, built by the Orsinis with portico on three sides.

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