When King Duncan arrives at Inverness, accompanied by Banquo and the rest of his party, the King admires Macbeth's castle, commenting upon its pleasant location and climate:
This castle hath a pleasant seat; the air
Nimbly and sweetly recommends itself
Unto our gentle senses.
Duncan's reference to "gentle senses" means that he finds this place not only pleasant, but soothing. Banquo then confirms Duncan's reaction to Inverness, noting that the air is "delicate."
This brief exchange between Duncan and Banquo shows that the King trusts Macbeth and views his castle as a place of rest and renewal. Duncan's judgment is unsound in this regard. Macbeth has deceived Duncan, managing to conceal his treachery, just as the Thane of Cawdor had fooled Duncan before betraying his trust. Banquo still trusts Macbeth at this time, although Macbeth's reaction to the witches' prophecies has puzzled him. The inclusion of this conversation introduces strong dramatic irony to the scene since the audience is well aware of Macbeth and Lady Macbeth's terrible intentions.
In Act I, Scene VI, King Duncan describes Inverness Castle in very positive terms. He says that it has a "pleasant" feel about it, for example, and that the air "sweetly" appeals to his senses. This judgment implies a couple of things about King Duncan. Firstly, that he trusts Macbeth and believes in his loyalty and, secondly, that he has no idea about the prophesies made by the witches in which it was stated that Macbeth would become king.
It is interesting that Banquo encourages King Duncan's positive view of Inverness Castle since he was present at Macbeth's encounter with the witches. As such, it implies that he does believe that Macbeth will harm the king, despite his obvious ambition. Moreover, it suggests that Banquo does not suspect that Inverness, such a welcoming environment, will, in fact, be the scene of a terrible murder.