Why did polygonal bastions replace rounded towers in many fortresses?

Polygonal bastions replaced rounded towers in fortification primarily to enhance defensive effectiveness against artillery and infantry assaults. While rounded towers were initially favored for their perceived structural strength, they created "dead space" at their base where attackers could hide, shielded from the defenders' line of sight and fire.1

The transition to polygonal bastions provided several critical military advantages:

• Elimination of Dead Space: The angular design removed the blind spots inherent in circular towers, allowing defenders to maintain clear, uninterrupted fields of view and fire along the fortress walls and ditches.1
• Improved Defensive Fire: Bastions featured two faces and two flanks, which permitted musketeers and gunners to sweep the faces of adjacent bastions and the connecting curtain walls with flanking fire.21
• Increased Firepower: The long, straight sections of the polygonal walls provided more space to mount larger defensive cannon batteries compared to the limited surface area of rounded towers.1

These changes were part of a broader shift toward low-profile, earth-reinforced designs that better withstood the destructive power of gunpowder weaponry.21