Cities have been fortified ever since they emerged in prehistoric times. Topography and natural objects, augmented by human-made defences like stone walls or timber palisades, were used to provide some form of natural protection against attack.
Up to the 15th century, the medieval castle walls and moat were sufficient defence against attackers. But this changed with the introduction of gunpowder and the cannon.
Constantinople fell to the Turks in 1453: it’s three-tiered defensive walls crumbled under the power of a cannon capable of firing projectiles exceeding 800 pounds in weight. The old fortifications became vulnerable, so new designs were introduced by Italian military engineers.
The new fort layout adopted a star-shaped form; the cannons and muskets of the defenders were brought to the outer-most positions, at the tips of the stellar fortification, to direct fire to attacking forces. The rest of the city would lie beyond the reach of the enemy's guns. Instead of the simple vertical castle walls were bastions, - a sharp, jagged pattern of earthworks battered to withstand cannon fire.
A prime example is Palmanova, in north-eastern Italy, close to the border with Slovenia, designed by Vincenzo Scamozzi (1552 - 1616). In his “L'Idea Della Architettura Universale”, (1615), he proposed that the “City should be not result of nature but product of planning".
Source: The University of Melbourne, FACULTY OF ARCHITECTURE, BUILDING AND PLANNING, CULTURE & HISTORY of URBAN PLAN Lecture Notes ©1999 C.M.Gutjahr