When I built my first pen, I made the walls pretty tall and filled up the inside of the pen. The height of the walls on the inside ended up being 18 inches, and the outside height was about 24. I also made sure to leave small gaps between the boards making it easy for the water to drain out the sides.
I'm planning a new pen for the new house I'm in now. It has to go around a corner of the house. Should be interesting! - lol!
We had a hail storm here for about 20 minutes last week or the week before, can't remember exactly when. The stones were small, maybe the size of a marble at best. It's a very rare thing here in NJ.