Situated on the slopes of Mount Morrone dominating the underlying village, the "castrum populi" was of great strategic importance as it oversaw the obliged passage of the "gorges of Popoli". It was built in 1015 by the Bishop Tidofo during the period of fortification of the villages for defensive purposes. Triangle-shaped, a pentagonal tower-strut stands out at the top (so called “male" of the castle) and two other towers in the lower part of the "castrum populi". These three towers have always represented over the centuries the symbol of the city of Popoli, joined one another by three boundary walls. At the end of the fifteenth century, Count Restaino Cantelmo ordered the building of the tower, the so-called "Round Tower", put beside two other bodies. The type of castle enclosure is widespread in Abruzzo, as well as highly developed throughout Italy.