A defensive wall or rampart.
A defense or safeguard.
A breakwater.
Any means of defence or security.
The planking or plating along the sides of a nautical vessel above her gunwale that reduces the likelihood of seas washing over the gunwales and people being washed overboard.
To fortify something with a wall or rampart.
To provide protection of defense for something.
To weaken or work against; to hinder, sabotage.
To erode the base or foundation of something, e.g. by the action of water.
To dig underneath (something), to make a passage for destructive or military purposes; to sap.