A moat is a deep, broad ditch that surrounds a castle, fortification, building, or town for defensive purposes. The moat could be kept dry or filled with water, and in some cases, evolved into more extensive water structures like lakes, dams, and sluices. The primary purpose of a moat was to protect the castle from attack. As a defense mechanism, moats were very effective, making the process of tunneling under a castle nearly impossible. Moats were excavated immediately outside the castle walls to make them more difficult to access by siege weapons, such as battering rams and siege towers. If they were filled with water, moats could also prevent the digging of tunnels under the castle. The existence of a moat was a natural result of early methods of fortification by earthworks, for the ditch produced by the removal of earth to form a rampart made a valuable part of the defense system. When earthworks gave way to masonry walls, the moat was retained and became even more valuable than before, as it prevented moving towers or battering rams from being brought up to the ramparts until the moat had been filled. Castles without moats were vulnerable to attacks from below, since marauders often found the only way to take a castles inhabitants by surprise was to tunnel underneath the castle and attack from below.