Bulwark

1. n. The sides of a ship above the upper deck.
2. n. A rampart; a fortification; a bastion or outwork.
3. n. That which secures against an enemy, or defends from attack; any means of defense or protection.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.