The sea isn’t far and the fact that the castle overlooks the Via Francigena gave it an important strategic role.
A document of the 8th century proves that in the Lombard period, Aghinulfo Castle already stood here. Aghinulfo was probably a member of the Lombard court of Lucca.
In the late Middle Age the castle, although not continuously, was under the jurisdiction of Lucca, as Montagnoso was considered valuable for its position controlling the sea. Some renovation done on the structure probably dates back to this time.
Other significant modifications seem to date back to the 15th century and work also continued in the following centuries in a series of expansions and developments of the structures.
The last military occupation of the castle, with the relevant architectural modifications, dates back to 1945, when it was occupied by a Nazi garrison, becoming one of the strongholds of the Gothic Line.
The architectural complex consists of variuos elements: sturdy walls with towers encircling two large spaces at different heights: the uppermost space is occupied by the parade round, with a rampant walk supported by numerous arches.
There is also a keep consisting of a large building with an octagonal base (probably the oldest in the entire complex) joined to a circular tower (baluardo di San Paolino) by a wall.
The octagonal building, dating back presumably to the 12th century, seems to have been used for both military and residential purposes.
The complex layout of the castle and the numerous modifications applied over the years are the basis of the recent restoration, which aims at laying bare the different phases of construction using archaeological reading techniques on the different objects found as well as special planning solutions.