Why Did Castles Have Moats?
The purpose of a moat was primarily to protect the castle from attack. Although they’re usually depicted as wide, deep bodies of water, moats were often simply dry ditches. Some moats surrounded the castle itself, while other moats might have enclosed several buildings or even a small town.
As a form of defense, moats were quite effective. Castles without moats were vulnerable to attacks from below, since marauders often found the only way to take a castle’s inhabitants by surprise was to tunnel underneath the castle and attack from below. Moats, however, made the process of tunneling under a castle nearly impossible.
When moats were filled with water, they were usually deep enough to make it difficult for attackers to wade across. In addition to being difficult to swim with weapons, attackers would be reluctant to try swimming across because they would be too vulnerable to attack from castle guards.
Moats filled with water were usually supplied by a nearby source of water, such as a spring, lake, or river. Dams could be built that would control the level of water in the moat. While some fancy moats may have had stone sides, most moats had simple banks of earth left over from when they were dug.
One of the only successful ways to overcome the presence of a moat was to use portable bridges to span the moat. Of course, carrying portable bridges and putting them into place before trying to cross to besiege the castle still allowed plenty of time for castle guards to prepare to defend the castle.