After the fall of the Roman empire when invasions from Africa made life difficult for the local people, theys retreated to the promontory, for better defence. There the Byzantines fortified the settlement which was named Akropolis. In the late 6th century the Longobard invasion forced the Bishop of Paestum to take refuge in Agropoli, which became therefore the main centre of the surviving Byzantine territories in the area. In 882 the town fell to the Saracens, who turned it into a powerful stronghold; from there they set out to plunder the surrounding areas, until in 915 AD they were defeated and Agropoli returned under the jurisdiction of the bishops who ruled until the early 15th century.
In 1412, Pope Gregory XII granted the feudal territories of Agropoli and Castellabate to King Ladislaw of Durazzo. In 1436, King Alphonse of Aragon granted the fiefdoms of Agropoli and Castellabate to Giovanni Sanseverino Baron of Cilento. In 1445 the Numerazione dei Fuochi recorded total 202 families. From 1552 Agropoli passed was under different families: the D'Ayerbo of Aragon (1553), Grimaldi (after 1564), Arcella Caracciolo (1597), Mendoza (1607), the Filomarino princes of Roccadaspide (1626), Mastrillo (1650), Zattara, and finally the Sanfelice, Dukes of Laureana (1660), who ruled until the abolition of feudalism in 1806.
During the 19th century, Agropoli began to expand outside the medieval walls, but the old town has remained intact, together with most of the surrounding defensive walls and the 7th-century entrance gate.