Previously, the Macbeth castle was depicted as an idyllic sight: "This castle has a pleasant seat; the air / Nimbly and sweetly recommends itself / Unto our gentle senses" (Duncan, 1.6.1-3) or "This guest of summer, / The templehaunting martlet, does approve, / By his loved
mansionry, that the heaven's breath / Smells wooingly here" (Banquo, 1.6.3-6); but after the murder it turns immediately into the castle of horrors, into Hell itself.
In the plainest terms, the porter's scene tells us that Macbeth has made a hell of his castle, where his monument will be the maws of kites, though it could have been the temple-haunting martlet and his loved
mansionry.