A literal bulwark is a wall that protects and defends a place.
And a figurative bulwark is also a protector or defender; that is, it's someone or something that protects against something else in a way that reminds you of a large, sturdy wall.
Part of speech:
(Countable nouns, like "bottle," "piece," and "decision," are words for things that can be broken into exact units. You talk about "a bottle," "three pieces," and "many decisions."
Likewise, talk about one bulwark or multiple bulwarks.)
"Bulwark" is also a verb: "her confidence bulwarks her,"
"her confidence bulwarks her against fear;"
"she's bulwarked by (or with) confidence,"
"she's bulwarked by (or with) confidence against fear."
So, the other verb forms are "bulwarked" and "bulwarking."
How to use it:
Call something (or someone) a bulwark when it seems to stand firm, wall-like, protective and defensive, against the onslaught of anything bad.
Examples of figurative bulwarks include military presences, philosophies, beliefs, religions, strategies, approaches, knowledge, understanding, wisdom, ideals, powerful groups of people united by common beliefs, and even individual people--especially leaders who promote certain values.
Most of the time, we refer to a bulwark against something bad: a bulwark against hatred or extremism,
a bulwark against enemies or threats,
a bulwark against chaos or destruction,
a bulwark against corruption,
a bulwark against the effects of climate change,
a bulwark against corporate invasions of our privacy,
a bulwark against encroachments on our constitutional freedoms.
And we don't have to be so dire: we can also talk about bulwarks against forgetfulness, disorganization, embarrassing mispronunciations, etc.
Do we ever talk about bulwarks against good or neutral things? Sure, if we're being sarcastic or critical. "Their policies amount to a bulwark against voting rights." "They've built, and now they hide behind, a bulwark against the increasingly egalitarian world."
You can simply say that someone or something is a bulwark, or that it serves as a bulwark, casts itself as a bulwark, is seen as a bulwark, is prized as a bulwark, etc.
(In older texts, you'll see the phrase "bulwark of something," with that "something" being either the thing protected ["a bulwark of our liberties"] or the protector itself ["the bulwark of our laws"]. But this usage seems rare today.)
examples: Peer review stands as a bulwark against shady science.