Creating button behind an image.

I have a picture of a castle and the image is pretty big so, instead of having a next page button, I wanna have then users just click on the door of the castle to go into the site... How do i create the button, and furthermore how do i put it precise ly behind the doord part of the jpg. or at least make the whole image a link to the next page.

Make the image a imagemap with the door a link.