At each corner, circular towers had been erected, and these towers occasionally protruded outwardly from the walls, with arrow slits on each level to allow archers to shoot anybody approaching the curtain wall. These are known as corbelled-out towers.
The main purpose of the circular tower is that it gives the illusion of greater height to any watcher looking out over the landscape. This makes an enemy target of interest all the more dangerous because they won't know how high their attacker is. The addition of these towers also adds strength to the castle's corner defences because there's no part of the circuit wall where a person could hide behind cover or jump down from a higher level onto those below. A defender in such a position would be easy prey for an attacking soldier armed with a mace or battle axe.
In addition to this, the sides of many castles feature small windows, usually round, sometimes arranged in groups (called "peepholes"). These were often only as large as a man's face but seemed huge then, since you were standing outside. They allowed the guards inside to see who was coming and going on the other side of the wall while being hidden themselves.
Finally, some castles have large wooden doors set into the side of the circuit wall.
Turrets were initially employed for protection in castles and other huge structures. Turrets, as opposed to towers, are curving chambers erected into a building, whereas towers begin at ground level. These early turrets had little openings in them for soldiers to launch arrows through. They could also be used to store weapons and supplies. The word comes from the Turkish tur rets, meaning "little tower."
As time went on, turrets evolved into more functional elements of architecture. For example, they could now be used as lookouts for defenders of the castle or town. They would also provide better viewing angles than lower floors of the building. Some historians believe that the use of turrets increased the survival rate of soldiers fighting in battles because they made it harder for their opponents to shoot them.
In homes, turrets were commonly used to hold clocks and other mechanical devices. They were also used as thermometers by placing an open container with hot or cold liquids inside them near an outside wall. The temperature change caused by sunlight passing through the walls in different seasons was used to tell the time of year.
Did you know that there are still modern turrets manufactured today? They're used for security purposes mostly.
Round towers were more difficult for attackers to bring down. They had no corners, unlike square towers, which toppled if holes were excavated beneath the foundations. These provided a safer way of launching arrows at the castle's assailants. Defenders also had an advantage because they could hide in these holes and shoot their enemies from a safe distance.
Towers were also better suited for defense than walls would have been. A tower's defenders could fire arrows or other projectiles out of its windows, while those trying to force the gate would be exposed to attack from any direction. A single defender could also fire arrows up into the air to flush out attackers who were approaching from above.
Finally, towers allowed the construction of higher defenses. As long as there was a platform on which to build, a tower's defender could shoot invaders off the platform or push them away with a club or other weapon. Or they could simply wait until someone climbed the stairs to attack them. Either way, attacks were prevented.
These are just some of the reasons why medieval castle builders used towers instead of walls. There were many others.