It's called a moat, and it is usually filled with water.
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.
A 'moat'
The water-filled trench around a castle is called a moat.
a Trench surrounding a castle is called a MOAT
"There was a giant trench around the castle".
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.
A water-filled trench around a castle is known as a "moat."
It is called a moat
A homonym for the trench around a castle could be "moat."
mote, moat
speck:motetrench around a castle:moat
A castle siege is when the enemy, surrounds the castle and they are going to attack and doesn't let anyone in or out or either forcing the castle residents to surrender or die from lack of food.