The speaker's middle brother is pining away for his freedom. As an outdoorsy, woodsy kind of guy, being imprisoned is hardest on him.
He stops eating after a while – not because the food is bad (after all, he's used to eating whatever he can get on hunting trips in the mountains), but because he's tired of living in captivity.
The middle brother is used to drinking milk from wild mountain goats, but now he only has water from the castle's moat.
Their bread was the same kind of bread that prisoners have cried over for the last thousand years – ever since people first started imprisoning their fellow humans like animals or "brutes."
But the quality of the food is not why the middle brother starts to waste away – he just can't stand being imprisoned.
The speaker's middle brother is the kind of guy who would have wasted away even if he'd lived in a palace – he'd have hated to be anywhere that he couldn't breathe fresh mountain air and run around freely.
So the speaker tells us bluntly – his brother died in prison.
The speaker sees that his brother was dying, but can't reach out to hold his hand or support his head, since they are all chained to individual pillars in their dungeon. The speaker tries to break through his chains, but can't.
After his brother dies, "they" (the jailers) come in to unlock the body from its chain.
They bury him in a "shallow grave" right there in the dungeon.
The speaker begs the jailers to bury his brother outside, where the sun would shine on his grave.
He figures it's what his free-spirited brother would have wanted, and he's afraid that his brother's spirit won't rest easy if he's buried in the dungeon.
But the jailers just laugh at him and bury his brother there in the dungeon, leaving his "empty chain" as the only marker of the grave.
The speaker calls that chain a "fitting monument" for his brother, since it was being chained that killed him.