An 8th-century document attests to the existence, even in the Lombard era, of Aghinulfo, an unknown character who seems to have been a functionary at the Lombard court in Lucca. The castle was built, albeit fitfully, in the late Middle Ages by the Lucchese government, which considered Montignoso a valuable place for control of the sea. The most recent military occupation of the castle was in 1945, when it was held by a Nazi garrison, who made a few architectural changes and turned it into one of the main strongholds along the Gothic Line.
There are many elements to the complex: a wide ring of walls, flanked by towers, encloses two spaces, both large but of different dimensions. The higher area contains the weapons-training ground and a patrol route, supported by numerous arches; a keep built on a large octagonal building (probably the oldest in the whole complex), which, via a walled courtyard, is joined to a circular tower, the bastion of San Paolino. The octagonal building, which is thought to date from the 1100s, seems to have had both military and residential functions over the centuries.