Round towers, also called drum towers, are more resistant to siege technology such as sappers and projectiles than square towers. The round front is more resistant than the straight side of a square tower, just as a load-bearing arch. This principle was already understood in antiquity.

Castles transitioned from square to round towers (often called drum towers) primarily due to military and architectural evolution in the 13th century. Square towers had a significant structural flaw: their corners were vulnerable to "mining" or "sapping," a siege tactic where attackers dug tunnels beneath a corner to collapse it. Round towers, lacking sharp angles, distributed the weight of the masonry more evenly—much like a load-bearing arch—making them far more resistant to structural failure and difficult to undermine. From a defensive standpoint, round towers eliminated the "dead zones" or blind spots created by corners, providing archers with a continuous 360-degree field of fire. Additionally, the curved surface was superior at deflecting projectiles from catapults and trebuchets, as a direct hit on a flat wall was more likely to cause a breach than a glancing blow on a rounded surface. This design change, often inspired by circular fortifications seen by Crusaders in the Holy Land, became a hallmark of advanced medieval fortification.