The moated manor house of Baddesley Clinton in Warwickshire, England

Moats were deep and wide water-filled trenches, to provide a barrier against attack upon castle ramparts or other fortifications. A siege tower needed to be brought up against a wall to be effective. A battering ram was also ineffective where a moat denied direct access. The word was adapted in Middle English from the French motte "mound, hillock" and was first applied to the central mound on which a fortification was erected (see Motte and bailey), and then came to be applied to the excavated ring, a "dry moat".

In the violent conditions of the 14th and 15th centuries, though defensive walling required a charter from the king, a moat round a manor house could deter all but the most determined intruders (illustration, right). See also Ightham Mote.

Often streams were diverted in the Middle Ages to fill the ditch. Moats required upkeep. They had to be dredged for debris which could potentially form a traversable bridge from one side to another.

Withdrawable bridges spanned moats in the Middle Ages. At first they were only simple wooden bridges that could easily be destroyed if an enemy was about to breach the fortifications. Later flying bridges and drawbridges were used for moat spans.

Moats sometimes had long woden spikes in them, to prevent enemies from swimming across.

While moats are commonly associated with European castles, they were also developed by North American Indians of the Mississippian culture as the outer defense of some fortified villages. The remains of a 16th century moat are still visible at the Parkin Archeological State Park in eastern Arkansas.

Moats rather than fences separate animals from spectators in many modern zoo installations. The structure, with a vertical outer retaining wall rising directly from the moat, is an extension of the Haha of landscape gardening.

In 2004 plans were suggested for a two-mile moat across the southern border of the Gaza Strip to prevent tunnelling from Egyptian territory to the border town of Rafah [1].