bulwark

1. (nautical) 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.Examples: "Entering that gable-ended Spouter-Inn, you found yourself in a wide, low, straggling entry with old-fashioned wainscots, reminding one of the bulwarks of some condemned old craft."

2. (figurative) Any means of defence or security.Examples: "The party stalwarts constitute the bulwark that ensures the president's term of office."; "Willing to upend the nation’s postwar role as a bulwark against authoritarianism, he promises to usher in a foreign policy rooted in “America First” transactionalism."figuratively