Sostruzioni sillane. (Roman substructures built under Sulla). Down along the descending steps, underneath arches and vaults, at the bottom of the tiny square of vicolo Leoncilli. They are only sometimes open inasmuch as they are part of a private building underneath the Bishoprick. It was built and added to from the third century to the first century B.C. It is thought to have been used as the private residence of the Longobard Dukes. Built out of large blocks of limestone, these substructures form a corner tunnel about thirty metres long. It is supported on the outside wall by strong arches resting on squared pillars and opened by splayed loop-holes. The barrel-vault is in concrete.