sentences:  After 270, the maintenance of an impenetrable solid frontier was abandoned by Constantine I in favor of a policy, whether deliberate or forced by circumstance, of "defense in depth".This called for the maintenance of a softer, deeper perimeter area of defense, with concentrated hard points throughout its depth.The idea was that any invading force of a sufficient size could penetrate the initial perimeter but in doing so with any element of surprise or rapid movement would be forced to leave several defended hard points (fortresses) to its rear, hampering its lines of supply and communications, and threatening surrounding of the force.