Star Fort

A star fort, or trace italienne, is a fortification in the style that evolved during the age of gunpowder, when the cannon came to dominate the battlefield, and was first seen in the mid-15th century in Italy.

Passive ring-shaped (enceinte) fortifications of the Medieval era proved vulnerable to damage or destruction by cannon fire, when it could be directed from outside against a perpendicular masonry wall. In addition, an attacking force that could get close to the wall was able to conduct undermining operations in relative safety, as the defenders could not shoot at them from nearby walls. In contrast, the star fortress was a very flat structure composed of many triangular bastions, specifically designed to cover each other, and a ditch. In order to counteract the cannon balls, defensive walls were made lower and thicker.

Although this made their climbing easier, the ditch was widened, so that attacking infantry was still exposed to fire from a higher elevation for a while, including enfilading fire from the bastions. The outer side of the ditch was usually provided with a glacis to deflect cannon balls aimed at the lower part of the main wall. Further structures, such as ravelins, tenailles, hornworks or crownworks, and even detached forts could be added to create complex outer works to further protect the main wall from artillery, and sometimes provide additional defensive positions. They were built of many materials, usually earth and brick, as brick does not shatter on impact from a cannonball as stone does.

Star fortifications were further developed in the late fifteenth and early sixteenth centuries primarily in response to the French invasion of the Italian peninsula. The French army was equipped with new cannon and bombards that were easily able to destroy traditional fortifications built in the Middle Ages. Star forts were employed by Michelangelo in the defensive earthworks of Florence, and refined in the sixteenth century by Baldassare Peruzzi and Scamozzi. The design spread out of Italy in the 1530s and 1540s.

It was employed heavily throughout Europe for the following three centuries. Italian engineers were heavily in demand throughout Europe to help build the new fortifications. The late-seventeenth-century architects Menno van Coehoorn and especially Vauban, Louis XIV's military engineer, are considered to have taken the form to its logical extreme. "Fortresses... acquired ravelins and redoubts, bonnettes and lunettes, tenailles and tenaillons, counterguards and crownworks and hornworks and curvettes and fausse brayes and scarps and cordons and banquettes and counterscarps..."

The star-shaped fortification had a formative influence on the patterning of the Renaissance ideal city: "The Renaissance was hypnotized by one city type which for a century and a half—from Filarete to Scamozzi—was impressed upon all utopian schemes: this is the star-shaped city." In the 19th century, the development of the explosive shell changed the nature of defensive fortifications. Elvas, in Portugal is considered by some to be the best surviving example of the Dutch school of fortifications.