In historical times, Proerna stood on the ruins of earlier settlements that had developed on the mound "Tapsi". Proerna lay on the northeast edge of Achaia Phthiotis, on the ancient road connecting the region of western Orthrys with the ancient city Pharsalos, as the city mentioned as Thessalian city between Thaumakoi and Farsalos. The town of the Classical and Hellenistic period developed primarily on the south side of the "Tapsi" mound. In the southeast part of the town was the sanctuary of Demeter Proernia. Proerna was fortified at the end of the 4th or at the beggining of the 3rd century B.C. The new enceince known now as "Gynaikokastro" stood upon the rocky promontory of Mount Narthakio north-east of the modern town Neo Monastiri. After the fortification was built, a significant part of the activities of the town was apparently transferred to within the walls. The south flank is preserved up to to the height of eight courses (4,20m.), the north flank has not been traced yet. If this flank has not been destroyed or buried under earth over the course of the year, this may mean that the protection afforded by the marsh on the north side was considered sufficient or that due to lack of time or money it was decided during the course of construction to limit the citadel to the higher of the two plateaux. If this is the case, the cross--wall across the saddle between the two peaks of the hill will have fulfilled the role of any outer flank. The absence of any gate or postern at the point where the cross-wall meets the west flank implies that the change in plan occurred when work had advanced. The wall, which is constructed of stone throughout , employs the local grey limestone. Where the ground is rocky and steep, the lowest courses of the wall rest on bedrock, which here is properly chiseled out. In more level areas, the wall rises on a wide socle of slabs, whose upper sides only are roughly hewn. Throughout the entirety of its trace the defensive wall is 1,50-2,00 m. in width, displays two faces constructed in irregular ashlar or isodomic trapezoidal, firmly - jointed masonry. The height of the courses ranges between 0,50 and 0,60 m. Not all rows are equal in height nor all their blocks identical in lenght. The occasional divergence in height of parts within the same course results in stepped horizontal joints. Furthermore, the upright joints are often set at an angle. Such interlocking increases the cohesion of the blocks employed in the face of the wall,, whilst also creating a striking effect. It also indicates that various groups of masons worked on the wall at the same time clearly with the aim of exploiting any available stone. Headers are inserted between sets of three stretchers, the headers being aligned vertically in every second alternating course. The headers penetrate the core of the wall containing earth, unhewn stones and chips. Open drains, whose purpose was to carry off the rainwater, are built into the outer face of the curtains. The construction principles and the development evident in the defensive wall at Proerna follow some system of technical specifications and are possibly the work of a particular group of engineers active at the end of the 4th century. The location and the construction of gates was one of the most crucial matters in defensive architecture. The most frequented road connecting the city with the countryside passed through the general gate of the wall. Two gates and two posterns of the fortification have been located so far. The gate at the southeast edge of the circuit was apparently the major one. Although it appears from the outside to be a simple gap 6,65 m. wide between two towers, it is provided with an inner L-shaped court. This was formed with the addition of a wall behind the outer double door opening. The second gate is in the west flank. It is a corridor 6,80 m. long and 6,50 m. wide, where the tower of the inner arm has been spared. Both posterns are straight openings in the south flank of the circuit, 1,30 m. wide. The fortification wall at Proerna is reinforced by rectangular towers erected across its exposed trace. Along with the gates they were the most prominent challenging and expensive enterprise. Apart from being a safe shelter for soldiers patrolling and defending and a storeroom for their arms. The towers of Proerna rose at regular intervals, ranging between 18 and 47 m. Changes in the direction of the wall trace as well as the gates and posterns were guarded by towers. As is to be expected towers are located at shorter intervals in the vicinity of the major gate. This particular gate was further flanked by additional towers. As in the case with other fortifications in Thessaly, half of each tower at Proerna is incorporated into the mass of the wall. However, the projecting part beyond the outer face of the curtain merely abuts, rather than being incorporated into its fabric. The towers that control gates or postrens are linked more originally to the circuit. They are slightly deeper and advance beyond the inner face of the curtain. This increassed width allowed the formation of an outdoor stone staircase leading to the sentry walk and to the chambers from the level upwards. These particular towers are 6 m. square, in contrast to their rectangular counterparts whose dimensions are 6,40x7,10 m. This technique and the choice of ashlar masonry is associated with the engineer's of Cassander and Demetrios Poliorcetes, successors of Alexander the Great, who struggled for supremacy in Greece during the last years of the 4th and early 3rd century B.C.