The poem starts with an image of an iron wall that looks unbeatable, then slowly takes that image apart. Iron rusts, the speaker says, and a wall that seems strong on its own will show its weaknesses once the world "comes down" on it. Strength, in other words, isn't permanent. It can wear away until it becomes something "low," and fear is all that's left.
In the last stanza the wall turns out to be the speaker. They once stood "iron strong" against their pain, but the pain won, and they describe themselves as lying where they fell. The mood is heavy, tired, and quietly defeated. The simple, steady rhymes and short lines make it sound calm, almost matter-of-fact, which makes the admission hurt more. The poem seems to be about the cost of trying to look unbreakable, and how exhausting that can become over time.