The castle in San Terenzo sits on top of a rocky spur known as La Bastia, which is at about fifteen meters from sea level, on the extreme western side of the bay overlooking the town.
It was built by the townspeople around an ancient watchtower and underwent several interventions throughout the ages, all the while maintaining its structural harmony.
The castle is erected on a pentagonal base with three circular towers at the corners which overlook the town with sturdy and thick defensives walls which enclose a second interior city wall.
One can get to the castle from Piazza Brusaca' by way of Via Meneghetti and the hill which leads to the entrance of this fortress.
Originally, this was built as a single ten meter tall square tower; subsequently a second wall was added, with an irregular pentagonal shape, equipped with circular lookout points on three corners.
A huge Barbacane reinforced the wall in front of the moat; now the moat has been filled but a slit from the north bulwark recalls its presence. Throughout the centuries, the castle took on the role of the defender of the town from the attacks coming from the sea, and more recently, teamed up with the even more imposing castle in Lerici to gain control of a substantial portion of this wide bay.
One of the first castle keepers was Giovanni of Mongiardino, already keeper of the Lerici castle in 1401, who settled himself in the castle of San Terenzo at the command of eight guards. Giacomo Rossi took over the role of castle keeper next, and fought against the Pisani, obtaining great praise from Simon Boccanegra during the period in which the castle belonged to the Republic of Genova in the Gulf of La Spezia.