The speaker cuts a window into the cellar, an ordinary, cramped, underground space, and the poem immediately turns it into something much larger. Once the window exists, the cellar is "full, a land," with its own French cook and a horse that is black but behaves like a perfect courtier. It feels like a small act of imagination opening up a whole private world, one that is a little grand and a little absurd.
The last line shifts the tone. "My daughter would rather marry a crow" is sudden and funny, but it also sounds like a father's complaint, as if the daughter has refused the elegant life he has imagined. The speaker seems proud and playful, and maybe slightly wounded. The mood is whimsical and dreamlike, with a hint of loneliness underneath, since the daughter prefers a wild, dark bird to the world he has built.