The article examines the evolution of the castle fortifications, which for each region and historical period had their own characteristic features and their relationship with the planning structure of small towns in Podillya. The features of the formation of fortification systems, which influenced the architectural and planning structure of the cities of Podillya at the turn of the XVI-XVII centuries, are revealed. The castle`s fortification and fortification system of the cities of Berezhany, Zhovkva, Brody, Stanislav, Zbarazh, Medzhybozh and others are considered. Attention is paid to the peculiarities of changes in the fortification system of cities of the structural-typological level and the use of the modern bastion system, which supplanted the fortifications of the medieval type. The influence of these changes on the intensity of development of small cities in Podillya in the period of the XVI-XVII centuries is analyzed. Attention is focused on the formative influence of the fortifications of castle structures on the architectural and planning structure of cities, which was often decisive at the last stage of the formation of castles and their fortification. The process of uniting the castle and the city is considered, which went through different stages of re-planning: from placing the castle separately, through the interaction of fortification and up to the stage of obtaining a general outline of the city fortifications. The role of castle fortifications in the system of fortifications of the small cities of Podillya is determined. The influence of the development of fortifications of castles and their merger with the defense structure of the city on the urban planning process of the cities of Podillya of XVI-XVII centuries is investigated.
 The formation of castle fortifications in the historical, architectural, town-planning aspects and their influence on the architectural and planning structure of the small cities of Podillya and significance of this influence for understanding the future development of small historical cities of Podillya are considered.