Design requires “contingency.” Rocks on a hillside can be explained by natural processes, but rocks stacked to make a uniform wall are contingent upon an intelligent designer or agent building the wall.
A second condition for design is “specified complexity,” as explained by William Dembski in his book Intelligent Design. Dembski explains that the wall we found, though not sophisticated by modern architectural standards, is obviously more complex than rocks randomly scattered throughout the woods. And, it was constructed for a specific purpose, namely to protect the soldiers who built it – so the wall has specified complexity.