The Byzantine texts particularly emphasize the role of Christoupolis as a fortified fortress: “the Christoupolis fortress, both as a carderon of the nature of the construction…and as it is a keystone, it is from Thrace and thirdly as a coastal being and from of Thalatta opportunely if it provided an alliance” (Ioannis Katakouzenos 1,4 Bonn Edition). The acropolis (the fortress) that dominates the top of the hill of the peninsula, at a height of about 70 meters, was rebuilt on the remains of the oldest Byzantine castle, in two phases: At the beginning of 1425, in view of the imminent attack of the Venetian fleet, the Ottomans built the ¨ new walls, apparently those sections that had been destroyed during the siege and occupation of Christoupolis (1390-1391). Around 1530 the walls were repaired again, in a larger and more thorough intervention. 

At this time, the outer enclosure of the acropolis was also built, with the addition of which the fortress expanded significantly and increased its defensive power. More specifically, the outer enclosure of the acropolis must have been built between the years 1530-1536, the time when Suleiman the Magnificent decided to strengthen the defense in several coastal positions (e.g. Thessaloniki-Lefkos Pyrgos). The outer enclosure connected the acropolis with the then half-destroyed perimeter walls of the peninsula and also fortified the lower side of the hill, towards the port. Here there are both entrance gates to the acropolis: The first (on the site of an older Byzantine one), is located at the end of today’s Kapsali street and is equipped with a scalding chamber. The second (from which the access is made today) is located at the end of Isidorou Street and was opened later. A third gate exists in the dividing wall of the two precincts, next to the cylindrical tower. It was the original entrance gate to the acropolis and the only one during the period 1425-1530, i.e. until the construction of the outer enclosure of the fortress.

The fortification of the Byzantine Christoupolis can be seen in several places under the later repairs of the walls. The central circular tower of the Acropolis is most likely from the Byzantine era – the existence of a fortified tower, the residence of the guard and suitable for projecting the last defense, was common in Byzantine Acropolises -, the tank, the two towers of the northern side, as well as the NW gate of the Acropolis with the zematistra, a defensive element of the Byzantine fortifications of the Greek area after the Frankish period. Of course, the original form of these buildings, which were adapted to the later fortification construction, remains unknown. An important fortification work from the beginning of the 14th century is also the “para Christoupolis wall”, which is still preserved in a monumental state in the north of Kavala. According to the information of the sources, the wall is the work of the emperor Andronikos II Palaiologos in 1302, with the aim of strengthening the defensive line of the straits controlled by the fortress of Christoupolis. However, Christoupoli was not a fortress only in the Byzantine years. In 1154 the Arab geographer Idrisi describes it not only as a strong fortress, but also as an important commercial port.

The fortress is mentioned very often in the sources after the end of the 12th century and it passed successively under the rule of the Lombards (1204), the Franks (1208), again the Byzantines and then the Turks (1391).