Castle walls transitioned from square to round primarily for defensive superiority during the Middle Ages. Square corners were a major structural weakness; they were easy for attackers to "mine" or tunnel under, causing the entire corner to collapse. A round tower, however, has no corners to pry apart and distributes the weight of the masonry more evenly, making it far more resistant to siege engines and later, early gunpowder artillery. Architecturally, round walls provide a wider field of fire for defenders; archers and crossbowmen in a circular turret have no "blind spots," whereas a square tower creates corners where attackers can hide from view. Furthermore, a round surface is more likely to cause a projectile—whether it’s a catapulted stone or a primitive cannonball—to glance off at an angle rather than hitting a flat surface with full force. By the 13th century, almost all sophisticated military engineering favored these "cylindrical" designs to ensure that the fortification could withstand both the physical pounding of a siege and the tactical maneuvers of an invading army.