城堡的防御墙保护居民免受敌人的攻击。

The castle's ___bulwark protected the residents from enemy attacks.

这位老将军是国家安全的坚强屏障。

The old general served as a ___bulwark for the nation's security.

海浪拍打着船舷，溅起的水花打湿了甲板。

The waves crashed against the ___bulwark, splashing water onto the deck.