Castles show how a fixed defence can enable offensive action even though the structure itself cannot move. An army that bypasses a fortress risks attacks against its rear areas and supply lines. Dover Castle supplies the example of a garrison threatening an invasion force that has landed in Kent and moved toward London. A defender can also hold an important area with comparatively few troops and send the rest of the force elsewhere. Fixed works therefore change force allocation rather than waiting passively for attack.
Notable Point · 31:21-32:33
How fixed defences release mobile forces
A fortress threatens an invader's rear and lets a defender hold important ground with fewer troops, freeing mobile forces for operations elsewhere.