For an outdoor enclosure the walls should be solid. Either wood, bricks or whatever. If the tort can see out of the enclosure they will usually pace the fence line trying to get out. They can also hurt themselves trying to get through wire. The top should be some sort of hinged or secured but removable top with screening or fencing to allow sunlight through but to keep other animals out. It is also a good idea to bury fencing under the ground to keep burrowing animals out of the enlcosure.