A star fort or trace italienne is a fortification in the style that evolved during the age of black powder, when cannons 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 contrast, the star fortress was a very flat structure composed of many triangular bastions, specifically designed to cover each other, and a ditch. Further structures such as ravelins, hornworks or crownworks, and detached forts could be added to create a complex symmetrical structure. Star fortifications were further developed in the late fifteenth and early sixteenth century in response, primarily, to the French invasion of the Italian peninsula. The French army was equipped with new cannons and bombards that were able to easily destroy traditional fortifications built in the Middle Ages. In order to counteract the power of the new weapons, defensive walls were made lower and thicker. They were built of many materials, usually earth and brick, as brick does not shatter on impact from a cannonball like stone does. Another important design modification was the bastions that characterized the new fortresses. In order to improve the defense of the fortress, covering fire had to be provided, often from multiple angles. The result was the development of "star"-shaped fortresses. They were employed by Michelangelo in the defensive earthworks of Florence, 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.