Populonia, a hamlet of Piombino, was in ancient times one of the largest and most important Etruscan cities and the only one to be built on the coast. Surrounded by crystal-clear sea, the borgo is in a dominant position, on one of the promontories that form the Gulf of Baratti. The ancient town was composed of two distinct districts: the high city on the Guardiola, the Telegraph and Castle hills, where the houses, temples and public buildings of the acropolis stood; the lower town, near the sea, housed the port, mercantile activities, and the metal works. The Castle of Populonia was conceived as a real fortified complex, equipped with a lookout tower and defensive walls. Built in the early 15th century, it was commissioned by the Appianis, Lords of Piombino. The Tower of Populonia was built during the 12th century by the Municipality of Pisa, and was a fundamental element in the maritime and land defensive system for centuries. The panorama stretches across the whole promontory to the islands of the Tuscan archipelago and Corsica. In the main square of the castle is the Church of Santa Croce, a small Romanesque church probably built between the 12th and 13th centuries. Over time the side chapels, the sacristy and the bell tower were added to the original plan.