We start this rock by filling the box half full of CaribSea™ gravel.
Any type will work, but I like the crushed coral.

step 1. Pour a layer of Aragocrete™ to form the base of the rock. This will look like a pancack

step 2. Place two cut off plastic bottles standing up on the base. I use the bottle opening like a cookie cutter to remove a plug the size of each bottle. These will be the towers of your castle. The bottles will be hollow because they sit on dry gravel.

step 3. Place the center bottle laying down between the the upright bottles.

step 4. cover the space betwwen the bottles with a layer of gravel, and remember that the gravel will become the openings around the next layer.