The present structure of the castle dates only partly from the Angevin period - specifically from the time of the famous "Vespro" War, which turned the area into a blood-bath at the end of the thirteenth and the beginning of the fourteenth centuries (1282-1302). The castle was then strengthened because, in common with Castellabate, it stood on the front line between the two armies.
The original fort must have been Byzantine. Its layout is usually referred to a triangular, but is actually in the form of a quadrilateral flattened at one end. Either, for strategic reasons, or because there was not enough space for more regular layout, the two northern walls were moved inwards. This give them the overall appearance of the third side of an imperfectly aligned triangle. The position of the round tower became the entrance gate and its associated defences.
However, in the early Medieval Norman period, there was possibly one square central tower within a walled enclosure, surrounded by larger outer walls which defended the town. Of these, hardly anything remains, just as there is little evidence of any further use in the following Suevan period.
Unlike the surrounding walls, which remained Norman or Suevan in style, the castle was continually modified as methods of warfare developed. Although part of the Angevin defensive wall and moat have survived, there is nothing left of the typical defensive structure of the period, with tower much higher than the surrounding walls, to hinder assault by besieging engines (helepoles).
As firearms came into use during the Aragonese period and mortars became particularly effective as siege weapons shortly before the middle of the fifteenth century, the walls were reduced in height and reinforced with sloping bases. This was done either to minimize injury to the defenders in the event of destruction of the towers by artillery fire, or to present a slanting surface to projectiles. The circular towers of Agropoli Castle are of this type, with sloping bases, battlements and other defences similar to the one in Castellabate, which dates from the first thirty years of the 12th century (during the lifetime of Abbot Costabile Gentilcore).
A covered corridor joined a series of embrasures in the towers and defensive walls. These covered the front and moat, so that crossbows and firearms could be used to defend against any attack from the north side. This was the only access to the castle, which had to be approached from the front, across a wooden drawbridge. There was also a deep, wide surrounding moat, which prevented besieging engines from getting too close, and forced attackers to cross an open area where they were exposed to defensive fire. Nowadays, there is an opening in the surrounding walls, allowing access to the castle. On the basis of information gained from recent excavations and the discovery of an embrasure two meters below its current level, it is clear that the original moat was much deeper.
There were look-out posts with narrow steps leading to the top of the towers, so that the sentries could keep watch on the base of the castle, as well as its main entrance.
The so-called Governor's Palace , being residential in character, appears less austere. The original building was for feudal-miitary purposes, but with the addition of the open gallery, for instance, it gradually began to look more like a place of residence, in a period when the exigencies of war were no longer the main consideration. It housed the current feudal lord and the baronial court at the time when the "Sapnish Peace" had replaced the turbulent period of Franco-Aragonese conflict and the baronial conspiracies led first by Antonello Sanseverino (1496), then by Ferrante (1552). After the abolition of the Barony of Cilento, which the latter represented, the castle was handed over to new feudal lords, starting with d'Ayerbo of Aragon.
Within the castle, the "French Hall" is also worthy of note; this spacious room was built on to part of the northern wall, when it was no longer needed for military purposes. It gets its name from an incident which took place in 1808, while a French garrison was occupying the governor's palace after it had been abandoned with the legal abolition of feudalism in 1806. The French had repaired an old cannon and used it to drive off a British man-of-war which had been pursuing a fully laden munitions ship, bound for Calabria under the command of a Maltese pirate appointed captain by Gioacchino Murat. There were also impressive fortified walls around the town, protected by high square towers, reminiscent of the Norman Conquest; these probably date, however, mainly from the strengthening of the fortifications ordered by Federico II in 1242. Access was via the main entrance gate, which was rebuilt during the seventeenth century by the Sanfelice family.
Their coat of arms was incorporated with those of the Delli Monti family, thereby preserving the name which would otherwise have become extinct on the death of Gian Francesco's wife. In 1807, the walls were apparently still intact on the side facing Lecina and the Bay of Paestum, but later in the century, a section was removed to make way for the new access road to the town. During the "Vespro" War, Agropoli Castle, in common with other along the Salerno coast as far as Policastro, was under the command of count Tommaso Sanseverino. Despite attempts to hold on to it, his wife, Margherita di Chiaromonte, was obliged, on the orders of King Charles II of Anjou and with the governor's approval, to hand it over to the Bishop of Capaccio. In 1412, Gregory II sold it to the King Ladislas, then the following years, it was ceded to the Sanseverino family, who held in until 1552. In that year, Ferrante's rebellion resulted in its confiscation and subsequent concession to other feudal lords. The last of these was Delli Monti Sanfelice family, who left it when feudalism ended in 1806, but do not appear to have handed it over officially. The castle was then used as a military barracks, and in 1810, it was entrusted to the major of Agropoli. However, much of it continued to be used by military engineers, against the wishes of Duke Gennaro. When he died in 1837, his heirs either sold or gave the castle to Ottavio Corasio, who, in turn, sold it to the Del Vecchio family. Meanwhile, in 1842, the War Department decided that the castle would cost far too much to restore and took no further interest in it.

(source: GuidAgropoli)