A moat was a ditch dug outside of a wall to defend against attacks. In some cases, the moat was filled with water. It was very effective at making an attacker’s life difficult, as it prevented easy access to the walls by men or siege equipment. It also made the process of digging a tunnel under the walls much harder. To build a bridge or fill the moat took a considerable amount of time, but was necessary if the attacker wanted to assault the fortress. This gave the defenders more time to call for reinforcements or harass their enemy.