Bulwark

bulwark [BOOL-wərk]
n. any protection against external danger, injury, or annoyance; any person or thing giving strong support or encouragement in time of need, danger, or doubt

[From Old Norse bole ("the trunk of a tree") + werk ("work, as an engineering structure")]

In its literal sense, a bulwark is "a wall or embankment raised as a defensive fortification; a rampart." But in literature and common parlance, it tends to be used in its figurative sense (as I define it above), in the same way "safeguard" and "defense" are used figuratively.

Franklin Roosevelt, for example, famously said:

The only sure bulwark of continuing liberty is a government strong enough to protect the interests of the people, and a people strong enough and well enough informed to maintain its sovereign control over its government.

John L. Lewis, president of the United Mine Workers from 1920-60, called unionized labor a safeguard against communist influence:

The organized workers of America, free in their industrial life, conscious partners of production, secure in their homes, and enjoying a decent standard of living, will prove the finest bulwark against intrusion of alien doctrines of government.

In his 1964 acceptance speech at the Republican Convention in San Francisco, Barry Goldwater called the North Atlantic Treaty Organization our greatest source of protection:

Now failure cements the wall of shame in Berlin; failures blot the sands of shame at the Bay of Pigs; failures marked the slow death of freedom in Laos; failures infest the jungles of Vietnam; and failures haunt the houses of our once great alliances and undermine the greatest bulwark ever erected by free nations, the NATO community.