The study of 17 forts in the forest-steppe zone of Western Siberia shows that early medieval defensive structures included escarps, 1.0–1.5 m high earthen ramparts, simple fences and ditches 3.0–3.5 m wide and 1.0–1.5 m deep. Overall, this indicates a decline of fortification traditions as compared to the Early Iron Ages. Smaller houses, lower mounds, and poorer funerary offerings during the historical Migration Period attest to a more primitive lifestyle. The decline was caused by the dominance of conquerors from the north. From 500 to 800 AD, this tendency still prevailed in the Irtysh basin because of the intrusion of small groups of hunters and gatherers. On the contrary, in the Tobol and Ishim area, defensive structures became more sophisticated and included wooden wall fortifi cations, watch and gate towers, underground passages, and wood-and-earth structures, which were 1.5–2 times larger. The rise of fortifi cations in westernmost Siberia was caused by the development of an agrarian economy, by long-distance trade, specifically with people of the Kama region and Central Asia, and by greater social and economic stratifi cation. At the same time, standards of fortification size and complexity had not yet been developed, and the key factor affecting the layout was the topography of the terrain. In 700–900 AD, small forts had strong walls only in “central” and specialized settlements.