a Trench surrounding a castle is called a MOAT
A homonym for the trench around a castle could be "moat."
A water-filled trench around a castle is known as a "moat."
A homophone for a speck is "spec". A homophone for "trench around a castle" could be "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.
"There was a giant trench around the castle".
speck:motetrench around a castle:moat
Herbert Trench's birth name is Frederick Herbert Trench.
A 'moat'
A drawbridge allows welcome guests into a castle to cross the moat(a trench of water that goes around the perimiter of the castle)
It's called a moat, and it is usually filled with water.
aleutian trench
Mariana's Trench