护城河
a deep, wide ditch surrounding a castle, fort, or town, typically filled with water and intended as a defense against attack
:
• The ancient castle was protected by a wide moat.
古老的城堡被一条宽阔的护城河保护着。
• They had to cross the moat to enter the fortress.
他们必须穿过护城河才能进入堡垒。