Why an invading army cannot simply bypass a fort

Why must an invading force attack castles instead of going around them?

Because an untaken fort leaves organized troops behind the advance to raid supply lines, forces the attacker to detach men, and often controls the same strategic site as a nearby town or crossing.

A castle protects more than the ground within direct weapon range. Forts serve as garrisons and rallying points from which mounted or foot troops can attack an invader's vulnerable logistics. Bypassing one therefore adds escorts, supply points, and risk behind the advancing army, while besieging it consumes time and combat power. Medieval armies carried carts, food, and support personnel, so living off the land did not remove their dependence on supplies. Towns also grew around fortified sites: Newcastle and York illustrate how castles and settlements could occupy the same strategically useful places.