A vital item to have in your arsenal of training equipment is a round pen. A lot of horse really can't be trained unless you use it, particularly when they are extremely stubborn or willful.
Round pens come in various diameters, usually 50 or 60 feet, and the gate which opens to permit entry also closes so your horse can't get out and away from you. This makes it a somewhat small, circular pen where you can train a horse most effectively.
Because of the size, if you stay in the middle you can usually reach out to the horse no matter where he is in the pen, especially with the aid of a training whip. The horse cannot be disobedient because you can reach him at any point.
Because the pen is round, once you have the horse in motion, he can keep going for whatever length of time you require. There are no corners for him to get stopped in. This is a huge advantage when you are working on commands and insisting that your horse stay in a gait until you give a command to change.
Additionally, due to the circular shape of the pen, the horse is forced to bend slightly while working along the perimeter and that is excellent exercise for him. If your horse primarily rides on trails, he can get inflexible since trails tend to be a lot of straight-aways, not much bending required. Horses need to be able to bend while working, in order to stay supple and flexible, which is good for their joints.
Because the round pen is too small to use for any activity other than work, the horse associates it with working and tends to be more focused while in it. This could be because of the size too, but for whatever reason, having your horse focused on a lesson can be a very nice change sometimes!
For all of the above reasons, the round pen is a wonderful place to train horses and an indispensable piece of horse barn equipment. If you haven't got one now, get one and before long you will find it difficult to imagine training horses without one!