The frontiers of Rome were never expected to stop tribes from migrating or armies from invading, and while a frontier protected by a palisade or stone wall would help curb cattle-raiders and the incursions of other small groups, Stephen Johnson (2004) Hadrian's Wall, Sterling Publishing Company, Inc, 128 pages, the economic viability of constructing and keeping guarded a wall 72 miles long along a sparsely populated border to stop small-scale raiding is dubious.