This study focuses on the southern line of wooden defensive structures (palisade, platform, and two towers) at Fort Umrevinsky (first third of the 18th century), based on the findings of archaeological excavations. Continuous development in this borderline fort are reconstructed over a period of 30 years. Initially, during the era of Peter the Great, Fort Umrevinsky was a regular, subrectangular fortification, enclosed by a palisade, and somewhat similar to a field redoubt. A few decades later, two towers were built on pile foundations at the corners of the palisade enclosure on the fort’s southern face. One of them was subquadratic, the other subrectangular in plan view. As a result, Fort Umrevinsky became a bastion-type fortification. The strengthening of the southern face was motivated by the presence of gates in the palisade wall between the towers, by the proximity of transportation routes (roads and waterways), and by the fact that fortifications were arranged parallel to the borderline. Fortification changes in the 1730s were caused by a number of factors. These included the spread of European fortification principles to Siberia, the political situation in southwestern Siberia, and the beginning of large-scale military engineering works in the region. The southern line of wooden fortifications at Fort Umrevinsky helps to estimate the number of towers there.