The article examines the evolution of theoretical concepts of the defensive architecture over 1700 years, from the mid-3rd century BCE till the mid-15th century. The oldest surviving treatise, in which a significant place is given to fortification, is the work by Philo of Byzantium (mid-3rd century BCE). In the article Philo's views are compared with the Vitruvius' views on the military architecture (second half of the 1st century BCE) and those of Vegetius (the turn of the 4th and 5th centuries CE). After a long break, just in the 15th century, a new architectural treatise by Leon Battista Alberti ‘De re aedificatoria' appears. The author of another treatise on architecture (‘Libro architettonico') observed in this article is Antonio di Pietro Averlino, better known as Filarete. The article represents the first part of the research, in which a comparison of the theoretical views of those authors on the form of plans for defensive structures is made. The author of the article highlights strategic and tactical concepts in the approaches of the authors of the treatises. The first one implies the location of the entire city on the ground, the plan of fortifications as a whole. The second one focuses on the individual characteristics of the defensive architecture (location of towers, curtains, gates, and so on). Philo of Byzantium pays a lot of attention to the tactical concept, completely ignoring the general plan of the city walls. The article discusses in detail all six variants of the outline of fortifications on the ground given by Philo. Vitruvius is a supporter of the strategic concept and stands for a round city. Vegetius follows rather Philo and refuses to support Vitruvius' position. Alberti is not strategically following Vitruvius. He sees the merits and demerits of both circular and polygonal planes. For the first time in the literary tradition, Filarete develops a star-shaped city plan.

The second part of the research, which is to follow, will be devoted to architectural forms.