and the forest, cunning cat, bristles its brittle fibers. (19-20)
We've never seen a forest compared to a cat before, but to compare it to a cat that "bristles" suggests that the animal's fur is standing up in a sign of fright or aggression (like what our neighbor's cat does when it sees our dog). This establishes an emotional mood in the poem, where violence and its effects are close at hand.
my horse for her house, my saddle for her mirror, my knife for her blanket. (26-28)
To Shmoop, what's interesting about this trade is what the speaker is trying to get rid of: a horse, saddle, and knife. Those things would be pretty handy to a soldier, but it seems that our speaker is rejecting them for a more peaceful way of life.
My friend, I come bleeding from the gates of Cabra. (29-30)
In these lines, the speaker invokes the violent past of his country, in which wars were fought between Christians and Muslims for control of the Spanish kingdoms. This included fighting at the village of Cabra, in southern Spain. Here, as the speaker bears this old wound, the poem reminds us that violence, even if committed in the distant past, remains painfully with us.