Medieval castles were typically military structures meant to secure a region for the lord.
Since the lord himself usually resided within such a castle, the security of a medieval castle was a key concern.
Among the vital defenses of such a castle was a moat.
A moat was a deep and wide ditch surrounding the entire castle.
It was often filled with water or set up wooden stakes.
A drawbridge was typically used to let people cross the moat to enter or leave the castle.
In the case of an attack, the drawbridge was raised and the attacking force had to contend with the moat before accessing the castle walls.
The medieval castle moat evolved from the early medieval motte-and-bailey castles in which the Normans excelled.
After the 1066 Norman Conquest of England, the motte-and-bailey castles dotted the entirety of England as Norman attempted to cement their control over the newly conquered territories.
In constructing such castles, the Normans would excavate earth and erect a steep mound with it, constructing the main keep or tower at the top of the mound.
Due to the earth excavated for the mound, a ditch was created at the bottom of the mound which became a formidable defensive structure, often filled with water for additional defense.
The word moat thus evolved from the French ‘motte’ and by later medieval period, it began to refer exclusively to the defensive ditch around a castle.
A medieval castle moat was built for purely defensive purposes.
The primary purpose was to keep the attackers away from the walls of a castle.
To accomplish this, the moats were often made to be up to 12 feet in width and nearly 30 feet in depth.
This expansive ditch was dug all around the castle’s external walls and was then filled with water or set up with wooden stakes.
Any enemy attacking the castle then had to contend with the moat before gaining access to the castle walls.
When filled with water, the moat was certain to stop the advance of an attacker.
The attacking force then had to swim through the water which was a significant problem.
As the enemy waded through the water, defenders on the castle walls could easily pick enemy soldiers with their arrows.
In case of wooden stakes, it became impossible for an enemy to launch a direct assault on horseback.
Consequently, men and horses had to make their way through the difficult ground during which they had to face attacks from castle walls.
A moat also effectively barred the enemy from undertaking tunneling against the castle walls.
Tunneling was often used in the medieval period to collapse a part of the castle walls by using fire and explosion.
A moat cut short any such possibility by being a ready body of water available to the castle and at the same time, leaving no direct access for the enemy to the castle walls.