In the picturesque setting of Capo San Marco, stands the church of San Giovanni di Sinis, built on the site of the ancient necropolis of the Phoenician-Punic city of Tharros. Built with local sandstone ashlars, this structure bears witness to a historical stratification that dates back to the Byzantine era (6th to 7th century). The façade is characterised by a division into three mirrors by means of round-headed arches, while the side elevations have mullioned windows with two lights dating back to the first manifestations of the Romanesque period (11th century). The building is distinguished by some unique architectural elements, including a square dome from the Byzantine period, connected to a circular one by means of pendentives, a construction solution also used in Constantinople. The simple interior features barrel-vaulted naves separated by two rows of three low arcades. Of particular significance is the presence of a rare early Christian baptismal font, which further enriches the historical and artistic value of this complex.