Moats were a relatively common defensive device. In retrospective it is not always easy to tell whether a trench was filled with water or not.

Trenches are, whilst being labor intensive, a defense easy to construct and maintain. Handily, the excavated earth could be used to build a rampart behind it. (Note that they varied in depth, from shallow in flatlands to metre-deep and hewn into stone in mountainous terrain.)

A moat is basically a flooded trench. This could be done as a temporary measure to improve defenses or as a permanent one. The water was procured by diverting a spring or small river into the moat. This would at times be maintained, making the moat "flowing", or be cut, making the moat a "standing" body of water. In the latter, fish could and would be held. (Whether the water became bracky is hard to tell. In any case, it would not be potable.)

Both moats and trenches are a rather suitable defensive device, making the approach to the wall harder, especially for siege equipment, such as ladders, as well as hindering unit movements. They also made it harder to dig under the walls.

Whilst in theory a trench/moat is easily crossed or filled with wood or earth, this becomes very challenging if the trench is defended. Trenches are only effective in combination with other defenses or defenders.