From a stand point of someone who is working in the picture pen I recommend one with two different ways to get in and out of the pen, makes it more effiecent if you have a way for them to enter and a way for them to exit
You also want different things on different sides of the pen, such as trees on one side and a holding lot on the other
Probably a level pen would be best but I also have never been in one with a hill
Jmo