Fire is the poem's one constant, but it keeps changing shape. First it is a "backward uniform" worn by buildings, an odd image that suggests flames dressing a building in reverse: instead of covering it, they strip it down to its bones. Then fire becomes something that speaks, telling the speaker that their question is "only a common cry." That line feels humbling. Whatever the speaker is asking, perhaps in grief, desire, or fear, fire reduces it to something everyone has felt and shouted before.
The last stanza shifts suddenly. "When you came O falcon / to my bed" addresses someone, or something, with the wild, hunting grace of a bird of prey. The sentence stops there, unfinished, which leaves it hanging. Judging by the imagery, the burning could be passion as much as destruction. The mood is intense and breathless, a little dangerous, and the speaker seems both exposed and awed. The poem never explains itself, and that gap is part of its heat.