A bulwark is a wall built for protection or defense. If you and your pals build a secret fort for girls only, you might want to build a bulwark to surround it — with a "No Boys Allowed" sign posted prominently.
A seawall that runs perpendicular to the shore is sometimes called a bulwark, because it defends the beach, not against invaders, but against beach erosion. Although the noun bulwark can refer to any fortified wall, it often specifically refers to an earthen wall, like a rampart. As a verb, bulwark means to fortify with a wall or to provide protection: "The volunteers used sandbags to bulwark the levee in an attempt to prevent the flooded river from overflowing its banks."