outer moat
Refers specifically to the outer defensive moat of a Japanese castle. Also used metaphorically in phrases like 外堀を埋める (to gradually close in on an objective by first dealing with peripheral matters).
江戸城の外堀は今では埋め立てられている。
The outer moat of Edo Castle has been filled in today.
交渉では、まず外堀を埋めるように小さな合意から始めた。
In the negotiations, we started with small agreements, as if filling in the outer moat first.
They built an outer moat around the castle to keep out enemy forces.
The soldiers crossed the outer moat by boat to approach the castle.