That will probably work, ok! The only thing is what type soil is under the gravel? Gravel tends to sink into the soil. If you have a clay soil it isn't as bad as a loam soil, it compacts so the gravel doesn't settle as much....but clay soils don't allow for water to percolate through, you'll...
Sorry I didn't see this sooner! I built a split level enclosure for one of my pens because the ground was not level and had a slope to it. I didn't have a blade for my tractor, so I couldn't level the ground before I built, unless i hired someone to come in and level it. I split the difference...