The fortification of the archaeological complex near the village Sidorovo – the largest hillfort of Khazar time on this territory is analyzed in the paper. Fortification consists of two lines of ditches and ramparts, located at a distance of 120 m from each other. The East side of the slope of the hillfort is fortified with the escarpment. Investigations produced on the inner and outer lines of fortification, showed that they were originated at different times. The small settlement was fortified by an internal line (perimeter 1,2–1,3 km). There was a need for the construction of another line of fortification in the process of its growth (the perimeter of at least 2.5 km). The time of the construction of ditches and ramparts internal lines – not earlier than the middle of the IX century – has been defined by the artefacts, found in their filling. Remains of wooden structures in the filling of the ditches indicate that the settlement had wooden or wood-earthen walls. The author considers the materials on fortifications of close type, existed in this territory in the XVII century, which contain data on the size of the garrison necessary for the normal defense of these fortresses. Comparison allows the author to suggest that there had to be a considerable military contingent on the hillfort near the village of Sidorovo. Otherwise, the construction of the fortifications described above would be meaningless.