My wife had a green tub, the same color as Castle Grayskull.

So I had to use it for something.

I used my Dremel Tool to cut away the sides of the tub to fit inside the castle.

(If you don't have an extra piece for your castle then don't cut away the side shown in the picture on the left)

Make sure to leave at least one leg for support.

I included slits in the side for the castle to hook into.

I also have a spare castle piece for the other side which I had to make a cut way in the tub for the hinge to fit in it.

Last put on the finishing touch by carving out stone brick shapes with the Dremel Tool.

This is the fun part!  Make sure the bricks are all different sizes, so it looks cool.