Fetching details from the dictionary...
outer moat (of a castle)
城の外堀には美しい蓮の花が咲いていた。
Beautiful lotus flowers were blooming in the outer moat of the castle.
外堀を埋めてから本丸を攻めるのが常道だ。
It is standard strategy to fill in the outer moat before attacking the main keep.
外堀沿いの桜並木は春の名所として知られる。
The cherry blossom trees along the outer moat are known as a famous spring spot.