The architectural complex is an outstanding example of Renaissance military architecture. It consists of a perimeter system of pillboxes that link three turrets, of a main area that includes the earlier tower built by Martin V, a rivellino and a wide surrounding moat. The flow of the Tiber, vitally important for the Borgo settlement, influenced the triangular shape of the Rocca, whose main function was to control and defend the river.
The strategic-military value of the castle was partly reduced in later years, by the construction of residential areas on the west side of the courtyard and by the grand staircase, which is attributed to Julius II (1503-1513) as indicated on the inscription over the door.
The new monumental staircase, separated into three ramps of stairs, has vaults and walls decorated with frescoes (the legend of Hercules) painted by artists from the school of Baldassare Peruzzi. In mid16th century, two events marked the decline of the castle: the siege by the Spanish Duke of Alba (1556) and the great flood of the Tiber (1557) which consequently moved the flow of the river to the north.
The changing conditions of the land together with the serious damage caused to the defence system brought about the decline of the castle, whose role in controlling customs duties was replaced first by Tor Boacciana (12th century) and then by Tor San Michele, said to be designed by Michelangelo.
In later centuries, the Papal authorities intervened carrying out routine maintenance until 1736 when Pius VI renovated the castle, repairing the damage done by the Spanish occupation, within a huge reclamation project aimed at using the salt marshes.
Systematic activity of excavation in the Roman town, first under Pius VII (1800-1823) and then under Pius IX (1846-1878) also brought about upgrading work on the building, that became the lodgings for the prisoners used to dig the sites and was also the first home for the archaeological finds.
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