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).
The outer moat of Edo Castle has been filled in today.
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.