The medieval fortress of Cetatea Albă (Asprokastron, Moncastro, Belgorod, Akkerman, etc., today Bilhorod Dnistrovskyi) located on the banks of the Dniester estuary includes the citadel with Cartesian plan and four prominent cylindrical towers surrounded by an outer wall belt. Plans similar to this citadel can be found both in the Roman-Byzantine, Oriental area and throughout Europe during the XIII-XV centuries. Several castella and quadriburgia with cylindrical corner towers (Pančevo, Dobra-„Saldum”, Hajdučka Vodenica, Brza Palanka, Rtkovo-„Glamija”, Milutinovac, Radujevac, Ljubičevac, Ušće Slatinske Reke, Kladovo, Podvis et al.) were built in the Balkan-Danube area. The Kula fortress in Bulgaria, Tulcea fortress in Dobrogea, Tatarbunar fortress and the New Fortress near Roman in Moldavia are built according to the same design. The citadel the Cetatea Albă fortress certainly imitates an external model, which belongs to the Byzantine-Oriental area. The model „emitter” could be the Roman-Byzantine castellum with a rectangular or square route, located in the Danube area or south of the Danube. The transfer of the model could be done with the help of masters of the Byzantine school of architecture invited by the beneficiary.