Dig down 10 or 12 inches and make a concrete, wire or wood barrier to keep the tort from digging out and to keeppredators from digging in. Have walls that are solid that go up 18 to 36." Then make a top that is secured with a latch and lock. I use a wood frame and wire mesh. It lets in the sun, and keeps out the predators. My 6x4 pen cost under $200. Now that it is built, I am proud, my Russian torts (pair) are loving it, but I wish I had made it larger. I will post picks soon.
Here is my pen:
Short of a bear, panther or human, they are safe. I inspect it regularly to make sure no rats or mice have chewed into the wood, and I kill ant mounds in the vicinity with Andro and make sure that no chemicals get near my pen.
I also water sealed the wood prior to making the pen so that the rain will not damage it for a long time.