The article describes a hazard caused by deformation of a brick vault located under the dungeons of the gate tower of Zamek Górny in Opole at the turn of the 13 th and the 14 th centuries, which was used as a school building until 2017. The hazard identified in 2013 was found on the ground level of the building in a communication route of the heavy traffic load. Visual inspections and tests have demonstrated that the existing situation was caused by civil works performed at the dungeon level

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... rly 6 years earlier due to an unconsidered decision on removing debris and a backfill which filled entirely the lower tower storeys. The above actions resulted in removal of the base of the floor 'laid on the ground' in the 19th century [2] [4] . In this article, an analysis was carried out to verify possibilities of unbelievable strength of the artificially formed flat vault, which was previously the floor supported on a debris and sand base.