It depends on who ever is making that moat. If it were you, You can make it as big as you like.
15 feet
Big walls and a wide moat.
a moat that is a wide ditch filled with water
Place both planks next to each other and slide across the moat then walk on them
Being wide and deep helped a moat to not dry out. In the UK, frequent rainfall topped up the moat. A nearby stream may have been diverted to flow into the moat, either using sluices to regulate the water level, or with an overflow channel.
A moat is a deep, wide, ditch, which is often filled with water.A moat is a deep trench that is filled with water and surrounds a castle or town. Its main function serves as a defense against attack.
The water-filled trench around a castle is called a moat.
I think that the 'trench' you are thinking of is a moat. You could say that it is a trench full of water that surrounds the castle. If you are not thinking of a moat then I am sorry.
The amount of water required to fill a moat depends on its dimensions, specifically its length, width, and depth. For example, a moat that is 100 meters long, 10 meters wide, and 2 meters deep would require approximately 2,000 cubic meters of water, which is equivalent to about 528,000 gallons. The specific volume can vary widely based on the moat’s design and purpose.
Companies that have a wide moat (dominate their competition), and are currently undervalued (selling for less than they should be).
A moat surrounded the castle.The moat was a form of defence for castles.
A water-filled trench around a castle is known as a "moat."