Everything in "Rise and Fall" is described as architecture. The speaker and another person, who seems to be a close friend or partner, were "built" on shared sorrow and late-night talks. The speaker kept catching "you" on the edge of the rooftop, which suggests they were always trying to save someone in crisis. Then the poem asks a harder question: what if the blueprint itself was faulty? Both of them, it says, were shaped by brain chemistry and imperfect drafts.
In the later stanzas, the speaker finds a way to grow, fixing their "design flaws" and learning, with outside help, to let people in. The other person doesn't manage this and slowly wears away. The ending is cold and a little guilty. The speaker calls "you" a beautiful relic, then turns away, "cutting the ribbons" and closing the windows. The mood is sad and tired, with relief and regret tangled together.