In the castle walls of Larissa may be distinguished phases belonging from the Mycenaean to the late Ottoman period. The lines of parts of the enclosures changed many times, according to the needs and the specific historical circumstances. The ancient material was utilized either in situ or as replacements in certain cases. The towers, of a square floor plan during the Byzantine period, sometimes changed their format after conversion in the Frankish, Venetian and Ottoman periods.
Today there exist two enclosures, one internal and one larger external one. The inner enclosure, the citadel of the castle, is almost hexagonal, and occupies the top of the hill. The outer enclosure, very expansive in the Byzantine period took in the citadel and plenty of space on its south side.
This account starts with the Citadel and the buildings of the Byzantine period. On the south side, where is now the bastion, there were recently discovered traces of an old, large quadrangular tower, which played a governing role being positioned beside the central gate, which is blocked today. A second small gate was set in the south part of the west wall. At the north end of the citadel are preserved the foundations of a large church, or rather a three-aisled basilica, dating between 6th and 10th centuries. Above its remains was founded a smaller single-nave church and this survives today in ruins. According to the founding inscription the church may be identified as one dedicated to the Virgin Mary, donated by Bishop Nikitas and built in 1174. Also inside the citadel there are two cisterns.
The large, oval outer perimeter follows the ancient line on the west and south sides. Periodically it was enhanced by three-sided and four-sided towers. In the southern part of the west side is revealed the last and the main gate of the Byzantine castle, whose interior would have had a vaulted passageway. The paved road outside that led to it has been identified. Among the buildings of the external enclosure the large vaulted cisterns are notable. Their size indicates the place was utilized by the urban population too, not merely that of the military garrison and the authorities.
In the walls, the Byzantine phases are to be found in the lower parts or in the gaps between the archaic phases. In some places they are identified as being of common rubble masonry with a few bricks at the joints, elsewhere they take the form of rows of rough-worked blocks of the local limestone, with pieces of brick inserted into their horizontal joints.
During the Frankish rule (1209-1389) and that of the Despotate of Mystra (1389-1394) repairs were made to the walls of the citadel (e.g. the northern wall) without any changes in their overall layout. To this period there dates a large and elongated building on the east side of the citadel, which had two floors. To the north it is also associated with, and butts onto, another two-storey building of tower-like form, which stood next to the Byzantine church. This large building was a secular structure, perhaps the home of the Frankish castellan. The great eastern cistern in the external enclosure possibly dates to the same period.
The First Period of Venetian rule (1394-1463) coincides with a watershed change in military technology: the introduction of firearms that caused radical changes in fortification-systems the world over. In Larissa the broad external enclosure was bisected, with the construction of a dividing wall, with two sections, linking the citadel to the east and west exterior perimeter walls. At the same time, the southern vulnerable part of the outer wall was slowly abandoned. The west part of the partition wall is supported by one quadrangular and one almond-shaped tower, one at either end. The eastern part is strengthened with one quadrilateral, one three-sided and one cylindrical tower. In these new towers for the first time at Argos appeared small cannon embrasures. The gateway for the northern part of the precinct opened at the end of the partition wall, near the citadel.
Concurrently the fortifications of the citadel too were renovated. On the inside the buildings damaged by the Turkish invasion of 1397 were levelled, and the ground surface heightened (this does not exist today because the excavations of the 20th century were taken down to the bedrock). The perimeter wall was also raised above the new, higher level and reinforced with new three-sided and cylindrical towers. In the towers were fashioned spaces for small firearms. After the infilling of the citadel, the Byzantine gate was sealed and a new one created in the new elevation, west of the old.
Finally, outside the Byzantine exterior north wall was constructed the complex of the twin towers, also with facilities for the placing of firearms.
During the Ottoman (Period One: 1463-1685 and Two: 1715-1821) and the Second Venetian (1686-1715) period between them, the defences of Larissa were enhanced. The Ottomans in 1467 erected a large cylindrical tower, as their conflict with the Venetians was continuous and Venetian Nafplio was located in the immediate vicinity. This tower dominated the castle, as is substantiated both by the engravings that immortalized it and also by its existing ruins. The Turkish traveller Evliya Çelebi informs us that the tower had eight floors, a stone cupola with a lead coating and was of a very great height; it could be compared only to that of the Galata tower in Instanbul. West of the central cylindrical tower they formed a little later a new low and circular tower-bastion, replacing the older tower.
There was also constructed a new wall, in the form of an outwork with a passageway, running from the cylindrical tower to the west tower of the eastern partition wall. By means of this construction there was created a closed ‘antechamber’ to the entrance of the castle, one that protected both the castle gates, that to the citadel and that to the northern external enclosure. In this outwork the new outer gate of the castle was opened, adjoining the cylindrical tower of the eastern partition. The gate took the form of vaulted passageway with a cobbled surface and benches on either side. It was secured with two doors: one on the outside and the other on the inside ends of the passage. A little later, beyond the gate was built yet another, second gate as a further defensive move. The space between the two gates was left uncovered, so that it could serve as a killing-field, being within range of shots from the roof of the tower and of the passage.
Among the other constructions of this period, the inside the castle saw the presence of a mosque.
The walls of the old external enclosure on the south side were by now dilapidated and of no purpose.
The castle remained in this form until 1700, when an explosion destroyed the large central cylindrical tower of the castle: it was being used as a powder magazine. In its place was built a little later the surviving bastion. Perhaps then, the enclosure of the southern yard was invested partly to strengthen and partly to accommodate the geometry of the bastion. This form the castle maintains today.