Most are familiar with the sight of a moat, traditionally dug around castles, and frequently filled with water. When the moat was first employed, it was meant to provide an additional defense of castles, towns, or large installments of people. A water filled moat made extremely difficult to storm a castle and or gain access to the walls of a fortress. Attacking armies could not simply climb the walls, and attempt to bring them down, because the moat proved a formidable obstruction. Further, attempts to fill in the moat or provide a crossing was often met with a volley of arrows, to discourage such attempts.
During the early Middle Ages, a moat might not be filled with water. It was still a trench deep enough to render it difficult for attackers to breach the walls of a building. A moat not filled with water is a dry moat. Later, most moats were filled with water. However, they were not, as some suppose, filled with alligators or sharks. It would have been virtually impossible to keep sharks alive in moat conditions; it’s very hard to keep them living even in today’s aquariums. Keeping alligators would also have proven impractical.
Most early versions of the moat did not have drawbridges, as one most often thinks. They did have bridges that could be removed easily at the approach of an enemy. In most cases, drawbridges were not employed until the late Middle Ages.
Though we commonly think of the moat in association with European Castles, Medieval Japan and China had impressive moat systems guarding cities and castles. Some Japanese cities would have not one but several moats. Some buildings might be built between some of the moats, but the vital parts of the city could be protected by as many as three moats. Sometimes these moats were the dry moat variety. Today, a few moats remain, like the one surrounding the Japanese Imperial Palace.
Some Native American tribes also built moats around central living areas, at least as far back as the 16th century. These provided some protection against raiding tribes or raiding Europeans. However, the introduction of the rifle to American Indians did render some moats useless, unless they were very wide.
Trenches dug during many wars work on the principal of the moat, though these are dry moats. Even today, a military force may dig large trenches to slow down an enemy, make motor transport impossible, or to keep tanks from crossing. A moat or trench can also be a useful place to hide during long battles.
Moats also frequently are used in zoos to keep animals in. These normally span a large, unjumpable distance, and are fairly deep. Fans of the computer game Zoo Tycoon, know digging a moat around dinosaur installments is an excellent way to keep large predators from escaping and eating the scientists.