The  Paliokastro, is one of the best preserved fortifications of antiquity in the Aegean. The wall is built at the eyebrow of the low hill above Mandraki, with large plinths of volcanic black stone, with a pseudo-building trapezoidal masonry system on both fronts. Inside there is a filling of raw stones and latip. Its average width is 3.50-3.80 m.

In the international literature is known the inscription which is engraved on the outside, north of the gate. The inscription states "THE PREFECTURE OF THE VILLAGE FIVE FEETS FROM THE WALL", ie defines the width of the public zone outside the fortification that had to remain free from any use for defensive reasons. The inscription is placed chronologically in the third quarter of the 4th c. π.X. and is an important element in the dating of the wall.

The surviving length of the wall in its southern part is 230, while in the eastern 80 m. It is reinforced by a series of rectangular towers, of which six are preserved on its south side and two on the east. In the southeast corner of the wall, a C-shaped recess is formed, where the only surviving gate of the wall is placed, in an invisible and sheltered position. For additional protection, the corner tower of the wall stands in front of it. The gate, 2.10 m wide, is preserved intact and is covered by elongated bricks, over 3 m long. Two ascending stairs are very well preserved along the eastern part of the wall. Of these, the southernmost is maintained at a height of 15 steps, while the second, at a distance of 40 m., Is preserved at a height of 18 steps. Similar climbing assemblies are arranged on the inner side of the southern part of the wall. Of these, only their base survives today.

The corner tower of the wall, opposite the gate, is 8.50 long and 9 m wide. The other towers have similar dimensions, which seem to have been solid, ie full at their lowest part. The fifth tower on the south side is presented with different masonry, which seems to be a later addition to an existing tower that may have collapsed. This tower is built with an isodome masonry system and with the insertion of wooden bricks at intervals between the streets. This characteristic masonry, which came from Greater Greece and is often found in fortifications of the Aegean and the coast of Asia Minor,  dates the tower to the end of the 4th or the beginning of the 3rd c. π.X. The wall surrounded the ancient city of Nisyros, which has not been excavated. The ruins of an early Christian basilica are preserved inside the wall.