The boards are just nailed up to the posts. The diameter of the pen creates small enough angles that the "fit" is not too bad. It has held up to much use for about three years now and the kids climb and sit on it all the time. To the best of my knowledge, the horse has never tested the boards.

The options in building such a pen are yours. We choose to have the board go up about 6 foot from the ground. The gaps between boards is also a variable. The higher the pen and the less space between boards, the more materials and the more work required to build. Also the diameter is up to you. Larger takes more materials and time, smaller less. But, you certainly can't go too small or it is useless. Ours is close to "minimum size" in my opinion. My wife wanted it a little higher and the gaps a little smaller than I had initially desired. She wanted to assure that the horse paid attention to her rather than the "world outside" when working with the horse. I believe it has worked out fine. From my research, if you have a large stud horse, they get kinda wild sometimes and you may need more secure posts, less or no gap between boards and more experience than we have with horses. I don't know that I could make a pen strong enough for a big, mean male horse. Most horses would be fine with the concept and design shown here.

The walk in is a great feature. I include the same concept on all fencing I do now, even on our "yard" fencing (just a personal like). Easy in, easy out. The "sign" above the pen was just a thing I wanted to do. It is routered out of the board and states "The Rosie Roundpen" as our horse is named Rosie.