Moats were a vital defense for Medieval Castles and were probably the best form of defense a castle could have. Enemy soldiers would have great difficulty overcoming a castle moat especially large and deep moats that slowed down the enemy and created a fantastic barrier between the defenders and attackers of the castle.
Most moats were then filled with water to make them impassable for an enemy. Natural moats were the most effective as they were commonly much deeper and wider than man-made moats that were typically 12 feet deep and 30 feet deep.
A medieval castle moat was typically constructed by first digging a ditch all around the castle walls. If the castle was located near a water body, the moat was then filled up with water.
If water was not readily available, the castle defenders would often choose to set up dry moats with obstacles such as rocks, wooden stakes excrement, and other obstacles in the wide ditch to make it as impassable for the attackers as possibly. The idea was to slow down the advancement of troops so that they could be picked off by the castles defending crossbow and longbow archers.
In some cases, a drawbridge was used for the same purpose which had the added advantage that it could be lifted in times of danger, effectively eliminating any passageway from or to the castle. A bridge was often erected to enable the castle inhabitants to cross the moat.
Such a bridge was then heavily guarded since it was the only entrance to the castle.