The witches give Macbeth two prophecies: one that he will be Thane of Cawdor and later King of Scotland, and that "none of women born shall harm Macbeth" "beware Macduff" and that he has nothing to fear until birnham wood is at his castle.
The messenger thought Birnam Wood was moving because he saw soldiers using tree branches as camouflage to approach Macbeth's castle. This coincided with the witches' prophecy that said Macbeth would be safe until Birnam Wood moved to Dunsinane.
The soldiers are on night watch at the castle in the opening of the play "Macbeth." They are accompanied by Banquo and Macbeth.
Because of his guilt in the death of Duncan and the witches prophesied that he would find the forest moving towards the castle and this scared him.
In Act 2, Scene 1 of Macbeth, the king, Duncan, is in a pleasant and gracious mood. He comments on the beauty of Macbeth's castle and expresses gratitude towards Lady Macbeth for hosting him.
Macbeth kills Duncan in his own castle at night time.
Macbeth's castle
There are a number of castles in the play: Forres, Inverness, Fife and Dunsinane. Inverness is actually Macbeth's castle.
In Act 1 they are living in Inverness, possibly in a castle or possibly in a large house. By Act 5 they are living at Dunsinane, which is a castle.
Macbeth's coronation
Glamis Castle.
MacDuff and Lennox knocked at the main entrance of Macbeth's castle to gain entry.