Round towers replaced square ones because corners were vulnerable to artillery

Evidence

Only in the Middle East, Europe, and parts of Asia did siege warfare escalate, and it’s here that fortresses evolved to their greatest size and complexity. The protruding towers and balconies we saw in Lachish selected for a means to breach walls at a distance, and by the time of the Hellenistic Greeks artillery was added to siege repertoires. Ballistas, wheeled contraptions with a spring-loaded giant sling or wooden spoon, hurled rocks several hundred yards. Giant, multiperson catapults fired iron-tipped spear-sized missiles. Boulders punched holes into walls and snapped battlements. The best hits were on towers, since the corners were more fragile than the rest. Hitting a corner just right could rip away chunks of the tower, and often the walls above the gash then collapsed. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 513 (#corroborates, via linked claim)

By the Middle Ages a new style of catapult was invented, the counterweight trebuchet. Trebuchets used gravity and leverage to hurl stones from a sling, and they surpassed all artillery weapons of their day. More accurate and powerful than ballistas, trebuchets could launch 350-pound boulders far enough to reach the inner fortress towers of most castles. Square corners once again proved vulnerable, and they all but disappeared. Round, cylindrical towers were less prone to collapse if they were struck directly. Even better, they were almost never hit directly, since their rounded edges deflected missile boulders to the sides. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 516 (#expands, via linked claim)

Fortification styles evolved in tandem with ever more effective artillery. Early structures relied on square-sided, protruding towers to provide flanking fire along the walls, but corners proved vulnerable to flying boulders. Below: Later castles had round towers to deflect artillery. Even when struck directly, these towers were less likely to shatter. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 972 (#corroborates, via linked claim)

Categorization

• XREF: connects to: Siege artillery evolved to exploit the fragility of fortress wall corners — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 513; Trebuchets made square castle towers disappear in favor of round ones — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 516; Explosive artillery ended the era of fortified structures above ground. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 974 (+1 more)

Related

• Trebuchets made square castle towers disappear in favor of round ones (similarity 0.94)
• Siege artillery evolved to exploit the fragility of fortress wall corners (similarity 0.93)
• Fortification style evolved in direct response to artillery effectiveness (similarity 0.86)
• Explosive artillery ended the era of fortified structures above ground. (similarity 0.84)
• Pushing siege weapons to longer range only drove artillery growth (similarity 0.84)
• Bombers evolved like castles, adding all-around defenses against attackers (similarity 0.82)
• City fortification walls evolved progressively from palisades to layered battlement systems by 1500 BCE. (similarity 0.79)
• Cannons ended the castle arms race, making thousands obsolete (similarity 0.79)
• Assyrian siege towers countered wall elevations by matching defender height and battering walls. (similarity 0.79)

Broader Connections

• Siege artillery evolved to exploit the fragility of fortress wall corners #corroborates (0.90) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 513
• Trebuchets made square castle towers disappear in favor of round ones #expands (0.80) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 516
• Fortification style evolved in direct response to artillery effectiveness #corroborates (0.70) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 972
• Explosive artillery ended the era of fortified structures above ground. #qualifies (0.50) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 974