This article presents the findings of a multidisciplinary study of ramparts of Ust-Tersyuk, the largest two-platform fort in the forest-steppe zone of the Trans-Urals. Excavations of the outer defensive line revealed a chronology of construction works from the early 4th to the late 13th centuries. Based on the analysis of paleosols, a succession of environments during the respective period was reconstructed: from 300 to 500 AD a typical forest-steppe prevailed; the humid and warm period was followed by a colder climate. Until the mid-9th century, the fort dwellers were people associated with the Bakalskaya culture, and later with those who left the Yudino and Makushino sites. Of the Early Bakalskaya culture fortifications, which surrounded the promontory, traces of a palisade remained. Later, on the western side, an earthen rampart with a base 3.8–4.0 m wide was built and a ditch 3 m wide was dug. On the front side a rampart with a base 5.0 m wide, with a ditch that was wider, was constructed. Remains of the Late Bakalskaya culture fortifications include traces of wooden wall fortifications and a watchtower, and at the side with the entrance, a renewed ditch 8.6 m wide and rampart built of sandy clay with a base 8 m wide. New fortifications were at least two times wider. Construction works continued periodically and included seasonal sodding of the ramparts. Later in the Middle Ages, the fortification was restored by building a wall of turf blocks on top of the eroded older structure. Excavation findings provide further knowledge of the cultural traditions and socio-political situation in which the ancestors of the native people of Western Siberia lived.