Many farms have round pens, and they are made from a variety of materials. Some are solid wood planks, others are pipe corrals. Some folks even use hay bales against a barn or run-in shed wall or fenceline. Whatever you use, you need to make sure that the containment and footing are safe for you and the horse.
The footing of a round pen gets churned up around the fenceline, so you need to bring in a mechanical harrow or use a rake to keep it level and evenly distributed. This will keep your horse focused on you rather than the uneven, clumpy footing.
Keep in mind that you also need to have level, safe footing for the human in the middle, too!
If your footing is simply the dirt in your field that has been worn down over time by constant use, you might consider adding sand, wood chips or other footing to make your round pen more useful following rain or other inclement weather. If you have added footing, make sure and really observe horses traveling on it to make sure it is still providing the cushion and traction that you want.
Your round pen is often used to keep your horse's attention on you during work or exercise. If the fencing is see-through, this might make it more difficult to keep the horse's attention during a training session.
No matter what type of fencing you have around your round pen, it needs to be sturdy enough to handle bumping, crashing, kicking or run-away horses. The fence should also be safe in case the horse bucks, kicks or falls; you don't want the horse to get a leg trapped in the fence.
If your fence is constructed of wood, make sure to walk the perimeter regularly to check for broken boards, splinters or loose boards. If it is of other materials, check to ensure all the materials are still hooked together properly and securely.
Don't forget the gate and latch. This often is the weakest point of the round pen, and it's a place where horses will try the security of the enclosure since they know they came in that way. This gate and latch need to be secure, but easy to open in case the human needs out of the round pen in a hurry.