古代的城墙作为坚固的防御工事，保护城市免受入侵。

The ancient city walls served as strong ___bulwarks against invasions.

健全的法律体系是社会稳定的重要保障。

A sound legal system is a crucial ___bulwark for social stability.

海浪猛烈拍打着船舷，溅起的水花洒落在甲板上。

The waves crashed against the ship's ___bulwarks, splashing water onto the deck.