Fortifications were sensible tools because they bought time, even though they were neither invulnerable nor impenetrable. Belgium could have fortified its German frontier while France built another strong line behind it, forcing an attacker to overcome two barriers in succession. The larger principle applies to maneuver warfare in any period, including Roman warfare: armies win by reaching the required place at the required time while preventing the enemy from doing the same. A defensive line slows the attacker and gives mobile forces time to react.
Chapter · 4:22-6:21
How fortifications buy time
Fortifications slow an attacker and give defenders time to react, even though no barrier is invulnerable.