After macbeth witnessed the apparitions, what were his plans to secure his kingship?

The First Apparition, 'an armed head', harped on Macbeth's fear about Macduff. The Second Apparition, 'a bloody child', wanted to deceive Macbeth with an assurance that 'none of woman born/Shall harm Macbeth'. Macbeth instantly thought of sparing Macduff's life; but soon thereafter, he reverted to his bloody design of killing Macduff to 'make assurance doubly sure'. At the end of the scene[IV.i], Macbeth was resolved to launch a deadly offensive on the castle of Macduff to liquidate the whole family of the thane of Fife. The Third Apparition, 'a child crowned', further equivocated to allure Macbeth with a false impression of invincibility:

Macbeth shall never vanquish'd be until

Great Birnam wood to high Dunsinane hill

Shall come against him.

 

From now on, Macbeth, deserted by most of the Scottish nobles, chose to confine himself to the castle of Dunsinane. He was banking a lot on the assurances of the Apparitions, desparately trying to protect his kingship from the base of Dunsinane.