In Act I, Scene 6 of "Macbeth," King Duncan, the Scottish lords, and their attendants arrive outside Macbeth's castle. With great situational irony, King Duncan praises the congenial atmosphere and pleasant environment, thanking Lady Macbeth for her hospitality:
This castle hath a pleasant seat; the air/Nimbly and sweetly recommends itself/Unto our gentle senses. (I,vi,1-3)
About this praise of Duncan, Banquo remarks,
This guest of summer,/The temple-haunting martlet, does approve/By his loved masionry that the heaven's breath/Smells wooingly here. (I,vi,3-6)
The martin is a bird that usually nests in churches, not castles. In Shakespeare's time martin was a slang term for a person who is easily deceived. However, while Banquo realizes that Macbeth's home is hardly congenial, there is situation irony present in his remark, as well. For, he, too, is unaware of the forthcoming bloody deed of Macbeth that follows the "False face [that] hide(s) what the false heart doth know"(I,vi, 30).
It is King Duncan who praises Macbeth's castle Inverness when he comes to meet him and his wife in the castle to have a supper, celebrating his nation's victory and Macbeth's glorious contribution to it.
His host is Lady Macbeth. This is an intensely ironic moment in the play because we as the audience are already aware of the dark plans of Macbeth and Lady Macbeth who are apparently playing host to Dunacn. The praise of the castle is all the more ironic as this is the place where he is going to die.
His praise likens the castle Inverness with the air of heaven and the divine breath that has made the atmosphere so fragrant. This heaven will undergo an ironic transformation or inversion into hell with Duncan's death round the corner. The other detail about the Martlet bird that has built her nest on the walls of the castle might apparently connote nurture and kinship but deep within even this detail reveals the deceptive nature of the castle. The bird is one that generally builds her nest in the church walls but in this case, she has been duped by Macbeth's church-like castle of evil.