The name of Istia (istia = island) and its importance in the Middle Ages was due to its strategic position, from which the city controlled the ford on the Ombrone river. Istia was also a residence of the bishop of Grosseto.
The castle contains the remains of the city walls. The outer wall has two gates: the Porta Grossetana
(XIII century), which still gives access to the city centre, and the Portaccia
The parish church dedicated to San Salvatore is entirely made of bricks and its façade features the coat of arms of Piccolomini.
The church first appeared in written texts in the XIII century.
In the church are a Romanesque font, probably obtained from a capital, a Madonna col Bambino
(the Virgin and Child), a wooden sculpture dating back to the beginning of the XV century and ascribed to the Sienese artist Domenico di Niccolò dei Cori, and a painting of the Madonna col Bambino
by Giovanni di Paolo, dating back to the half of the XV century.