Actually, defense in depth is a perfectly valid defensive plan. Concentric sets of walls were fairly common, consider what a typical castle town looked like. Byzantium/Constantinople was known for its three massive walls as well. Even if the outer wall is unmanned, it still slows down attackers while you rain all sorts of pleasant things down on them from the inner one.
Also, you could think of it as taking the palisade logs and planting them at an angle instead of vertically.