The Castle is easily discernible by its unmistakable circular plan with a double perimeter wall. At the centre, rising on an imposing scarp basement, are the remains of the old edifice. Even if the foundation date of the first castle nucleus is unknown, it can be safely assumed that it must already have been in existence in the early Middle Ages.
The Castle of Lari is accessed through a flight of 95 steps. About halfway up is a cistern dating from 1448, surmounted by the coats of arms of the Pitti and the Scali, former vicars of Lari. The present structure dates from the first half of the 17th century. The castle forecourt, surrounded by high scarped brick walls, is overlooked by a chapel and by the Palazzo Pretorio, whose façade is still decorated with the escutcheons of the various podestà who ruled Lari over the centuries.
The complex was used as a prison until World War II, when it was seriously damaged by bombings by both the Germans and the allied forces. A plan for the historical recovery of the building was launched in 1991.
A law court was active in the castle until 1962. The courtroom is now mainly used for lectures and civil marriages. The castle access courtyard is also available for the purpose.
At present days, the fortress hosts exhibitions, events and weddings.