The Walled-up Gate owes its name to the fact that it was closed at the beginning of the 16th century, when the fief was reconquered by the Caetani, namely Guglielmo Caetani. Its outline and round arch can be seen at Piazzale San Francesco, at the start of the Antero Vaj steps.
Originally, on this side of the medieval walls, beside the two hollow rectangular half-towers projecting from the wall face, there were two gates: the walled-up one towards the south-west corner, and a second about halfway along the circuit, of which all trace has been lost.
The walled-up gate, however, belongs to the Roman town of Circeii, since it connected the lower town both with the Acropolis above and with the road leading to the Promontorium Veneris, along the Via del Faro towards Valle Caduta.
The gate was linked by the cardo, the second most important street in the Roman town plan, to the Ancient Gate on the opposite side, from which the road descended towards Terracina. The particular position of the gates, from which the cross streets of the village began, intersecting the decumanus of the Roman town, is the only clue suggesting that the street grid was also planned rationally during the 13th century.