To protect their city from advancing invaders, citizens erected a made of boulders, brick, and wood.

Your answer would be BULWARK...is a defensive wall.

Hope that helps!!!

The correct answer of the given question above would be the third option: MATTRU JONG. When the rebels attacked Mogbwemo, Ismael Beah was away in another town called "Mattru Jong" with his brother, Junior & their friend Talloi. Hope this is the answer that you are looking for.