The revolution that took place in the Italian Renaissance was one of the most dramatic overturns in the architectural history. Strictly, there were two great revolutions: the first one was the turning towards the heritage of the classical antiquity, the second one was the new method of fortifying places. Today the first event has a long and extremely fruitful tradition of interpretation but the second one only calls for a research.
During the period of the late 15th — early 16th centuries the new fortification (it. “fortificazione аlla moderna”) was born. The event predestined the development of the fortification no less than for two centuries. At that time thick city walls and tall towers were replaced by thin curtinas and low bold platforms — the bastions that served for arrangement of batteries of artillery. Another important thing about the new fortification is that it implicated the perfect (from the Renaissance point of view) geometry, which meant the use of equilateral polygons as the base of their plan design. What is more, that perfection was about both pragmatism (engineering) and ideality — the two aspects that were bound up. That is verified by the material of the Renaissance treatises on architecture, fortification and mathematics (those by Francesco di Giorgio Martini, Pietro Cataneo, Niccolo Tartaglia, Buonaiuto Lorini, Francesco de Marchi, Vincenzo Scamozzi, Galasso Alghisi and others).
Unlike the 17th–18th centuries, when the fortification became a sphere independent of the “civil” architecture, the Renaissance was the period when fortresses, villas, palazzo and churches were designed and build by the same architects who passionately studied the ancient ruins and Vitruvius at the same time. The paper deals with the relations that bound together the new fortification and the key problems of the architectural thought of the Italian Renaissance. Among them there are: the ideal city, the very Pythagorean attention to the problem of numerical relations that reveal the correlation of the micro- and macrocosm; the utopia of the Platonian ideal state; harmonic proportions. All these ideas formed the intellectual space that gave birth to the fortification “alla