I did this the week before Christmas, with the help of my sister and mother. It took a total of 40 hours of work, 6 pounds of confectioner's sugar, and a disgusting amount of candy and cookies. Many more pictures, as well as recipes and instructions, are available here: [link]
By the way, it's all edible. Well, not quite all. The actual castle is built on a platform made of cardboard and empty quart-size yogurt containers. And the marshmallows in the back are over a cardboard slope. But everything else was food, including the hill in front (ice cream cones, oreos, and lots of chocolate).
2/14/06: Updated with a nicer looking collage.