The article gives a historical interpretation of a stakewall with an underground passageway found in the cen-tral part of the upper posad drawing on the materials from the excavation site in Oktyabrskaya Street (204 m2). The thick log wall consisted of vertical posts erected at the bottom of a specially dug ditch. The underground pas-sageway constituted a manway, starting on one side of the stakewall and ending on the other. Its ceiling and walls were covered with planks supported by low half-logs, thick planks and small logs. The plank ceiling of the under-ground tunnel was just below the base of the log wall, with the horizontal adit being so small that one could only crawl through it. In order to determine the absolute age of the stakewall, we carried out the dendrochronological and radiocarbon studies of its logs. For the purpose of identifying this object with one of those mentioned in writ-ten sources, we reconstructed the history of fortification construction and localisation by performing a detailed analysis of historical data and all known plans of the city. As a result, it was established that the wall found during the excavation in terms of its location and orientation better correlates with the building shown on S.U. Remezov’s plans, which was located in the central part of Trinity Cape and surrounded by a rectangular stakewall, rather than with the posad fortifications. On the plans of 1687 and 1688 from the Chorographic Drawing Book, the object in question was captioned as ‘prison’ and ‘prison yard’. This assumption allows us to date the log wall discovered in Oktyabrskaya Street at 1687, or, quite possibly, at an earlier time. This prison yard fence could be used after 1714 and, judging by the stratigraphic and planigraphic observations made at the excavation site in Oktyabrskaya Street, until the period of stone construction in the upper posad.