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 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. 

From Wikipedia