History of Circeo between the 4th and 9th century is extremely obscure. We may assume that the area partook in the events touching the rest of Latium, but only further excavations might shed some light on this lost history. Settlements in the area were probably harshly affected by Saracen pirates incursions, that Pope John VIII opposed in 877 together with the maritime republics of Gaeta, Naples and Amalfi.
The first information we have on what became of ancient Circeii date back to the 12th century, by which time the place was commonly known as Rocca Circea. We do not know when the first fortress was built or who did it, whether the Pope or the inhabitants of Terracina. Anyway, Pope Gelasius II wanted it to be trusted to the latter, between 1118 and 1119.
The mandate was however short-lived, and the administration of the fortress passed to Marino di Formosa and his sons, under circumstances we ignore. It is possible that Marino, whose family came from Terracina, was a captain sent in the area who seized the territory he had been trusted with. The 1132 agreement with Gaeta may confirm this hypothesis: this pact, which looked very much like a feudal investiture, could have protected the usurper from retaliation by Terracina, which was at the time an enemy of Gaeta. This new status was ephemeral as well: in 1138 Gaeta became part of the Norman Kingdom. Once again, it is hard to know what happened to Circeo.
At the beginning of the 13th century, Terracina rose up against the Frangipane family, conquering Rocca Traversa in 1202 and Rocca Circea in 1203. Pope Innocent III immediately intervened, ordering the fiefs to be given back. We ignore the result of the Pope's intervention, but in 1207 Terracina was again in charge of Rocca Circea, although under the Frangipane supervision.
In 1212 the Templar settlement began. After many difficulties, they managed to succeed to the monks of Grottaferrata at the church of S. Maria della Sorresca. After the events of the previous years, Innocent III probably wanted a more direct control over a highly strategic post.
Pope Gregory IX confirmed this choice, ordering the inhabitants of Terracina to build, in 1239, fortifications at Circeo, to be paid by the Papal States. The Pironti family ruled from 1259 until the end of the century. Over this period, the inhabitants of Rocca Circea fully became Terracina citizens.