The Romans discovered that walled fortresses were more easily defended if towers were built into the defensive walls. These towers made it easy to give covering fire for the walls.

Castles have tall towers primarily for defense, surveillance, and status. Architecturally, a high tower provided a superior vantage point, allowing defenders to spot advancing enemy armies or smoke from distant fires long before they reached the gates. During a siege, towers served as "fighting platforms" where archers and crossbowmen could fire downward with a significant range advantage; the height made it difficult for attackers to return fire accurately. Towers also eliminated "blind spots" along the castle walls; by extending outward and upward, they allowed defenders to shoot along the face of the wall at anyone trying to scale it. Furthermore, the "Keep" or "Great Tower" served as the final stronghold; if the outer walls were breached, the nobility could retreat into the tower, which was designed to be self-sufficient and easily defensible. Beyond utility, a tall tower was a powerful psychological symbol of the lord's wealth and authority, looming over the local landscape to remind the populace of their subservience. Over time, towers evolved from square shapes to round ones, which were more resistant to projectiles and "mining" attempts by enemies.