bulwark

Short definition from Wiktionary

Noun

1. 1
   A defensive wall or rampart.
2. 2
   A defense or safeguard.
3. 3
   A breakwater.
4. 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.
5. 5
   Any means of defence or security.

Verb

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