The citadel fortification consisted of earth embankments placed on different height levels, stone-block walled precinct and rectangular towers. The fortification elements were build in several construction stages. During a first stage, at the end of the IInd century BC or in the beginning of the Ist century BC the horseshoe-shaped embankment was built on the eastern, southern and western slopes as well as the rampart surrounding the plateau, and the first terraces. During the second construction phase, starting at the middle of the Ist century BC and later during the Ist century AD, were built the Hellenistic-type stone ramparts, as well as the temples with limestone plinths. During this phase, the fortification was destroyed by a powerful fire, most probably during the first Trajan war, in the years 101-102 AD. The last construction phase covered the period between the two Daco-Roman wars. The fortifications destroyed after the first war were rebuilt especially the embankment surrounding the plateau ant the firs terraces (“the red embankment”). The access gate of the stone wall was blocked using even a temple's plinths. In spite of these restorations, the stronghold was conquered again in 106 AD and put through fire.
The fortification system combines very well the traditional elements (earthworks) and foreign construction techniques (stone walls of Hellenistic inspiration). The traditional technique supposes digging a deep trench, the excavated soil being placed on one of the trench's edges as a high earthwork on which a palisade was placed. This is the technique used for the construction of the horseshoe-shaped, 340 m long rampart on the western, southern and eastern slopes of the hill. The fortress plateau and the first two terraces had a similar defensive system, the palisade being made of wooden poles connected with split tree trunks.
The Hellenistic inspiration wall was placed in the south-eastern side of the fortress, completing the fortification with the local earth rampart. The wall connected three towers having their ground level made of limestone blocks, it had it had buttresses on the southern side. Three more towers were built using the same technique, two of them being placed on the northern slope, where no fortification elements were found, another one being found in the south-western part, near the earth rampart.
The walls of the Costești fortress meet the most closely the original construction technique, being considered rather Hellenistic walls rather then Dacian walls (murus dacicus). The wall is made of limestone blocks places in assises (horizontal successive rows) on two paraments (parallel longitudinal rows) connected through transversal wooden beams fixed at the ends in special holes, carved into the blocks and the inner space infilled with an emplecton (rubble and earth). The blocks are finished (straightened) on five of the six sides, the interior one, towards the emplecton, being only roughly worked. From the construction are not missing the blocks fixed perpendicularly on the parament's direction and entering deeply into the emplecton (butise). Such a wall was 3 m wide and probably over 5 m high. Greek architects and masons recruited from the cities of the western Black Sea shore were, probably, also involved in this work.
The entrance in the fortress was on the south-eastern side, where are concentrated most of the fortification elements. The ancient road coming from the valley, first passed near the towers from the northern slope of the hill, built here namely in order to control over the acces and it went through the horseshoe earth rampart in the eastern side of the fortress. The road continued further on to the south, along the rampart, reaching the south-eastern side of the fortress where the stone wall was built with an opening near the central tower.
Two dwelling-towers (keeps) were built on the upper plateau: one placed near the northern extremity and the other one on the southern one. Both had a ground floor built of limestone blocks and an upper one made of mud bricks, the building had a massive tile roof. A watch tower and some wooden barracks were probably built between the two towers.
Four rectangular temples of the column alignment type were built inside the fortification and near it. Three of the temples were placed on the eastern terraces and protected by the existing fortification elements, while the fourth temple was situated on a terrace on the northern slope, near a tower. From the cult edifices, only the limestone plinths serving to support the wooden columns were preserved.
Civilian constructions/ facilities
A monumental staircase of limestone slabs led to the tower in the most southern part of the fortress plateau. The staircase was 3m wide and bordered by two limestone draining grooves. Another smaller stone staircase facilitated the access to the tower from the northern side of the plateau.
The problem of the water supply for the fortress was solved by digging water storage pits into the bedrock that would collect the rain water, since in the area there were no springs.
Traces of Dacian habitations were discovered in the lowland near Apa Gradistii, where the civilian settlement stood. On the southern end of the village, at Năpărţi, was discovered a house covering an area of almost 4 m2. The large amount of ashes and the lack of clay plastering may be proofs of a light material construction for this house. Another house from the actual village area was discovered at Laz. The house had a single room, probably rectangular. The wattle and daub walls supported a thatched or a shingle roof.