Final answer:
Bulwark means a strong defensive wall or barrier.
Step-by-step explanation:
The word ‘bulwark’ refers to a strong defensive wall or barrier. It is often used to denote a protective structure that is built to defend against attacks. In the context of the sentence, the men erected a bulwark near the opening to provide protection and security. This use of ‘bulwark’ emphasizes the idea of fortification and defense.