Step 3: Walls
The walls will be constructed out of the roofing tiles. Measure out how many you will need to make it all the way around. Be sure to account for an overlap when connecting two together, I used 1-2 bumps as an overlap. For the corners, drive a long piece of 1 1/2" PVC Pipe as a stake, it needs to be about 12 Inches longer than the height of the walls. But drive them in so they are about 2 inches less than the height of the walls. Cut the roofing tiles with the utility knife; it will take several passes before you make it all the way through. When you have the pieces cut, lay them down on the ground and use the 1/2" screws to attach any overlaps together(you should only need to use 2). Then place the complete walls in the pit you previously made, the bottoms of the walls will be below ground level. Attach* both ends to the PVC stakes in the corners. Do this for all the walls.
Optionally, you can place paving stones around the inside perimeter, to stop any attempts at digging out. I decided not to because the ground was so hard they wouldn't be able to dig through it anyway.
*When you're screwing through the roof tiles, be careful that you don't rip right through the tile itself