bulwark

noun

1. A defensive wall or rampart.

   “Let thouſands die, their ſlaughtered Carkaſſes Shal ſerue for walles and bulwarkes to the reſt:”

2. A defense or safeguard.

   “The royal navy of England hath ever been its greatest defence, […] the floating bulwark of the island.”

3. A breakwater.
4. 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.

   “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.”

5. (figuratively)Any means of defence or security.

   “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.”

verb

1. (transitive)To fortify something with a wall or rampart.
2. (transitive)To provide protection of defense for something.