Legions manning fortifications if entirely inside the forts cannot be hit.

These fort-manning legions cannot directly engage enemy legions but must fight by using the fortification strike potency.

If any part of legion is outside of the forts, the legion becomes vulnerable.

Forts have a maximum defensive cover based on their strength/size. This gives a maximum number of men it can shield.

If a fortification is besieged so its strength falls below the potency required to shield the size of legion manning, some of the legion will be exposed - manning no longer - and thus the legion becomes vulnerable.

Forts are an effective shield against melee/short-range attack between legions not manning forts but in the same location or within two localepoints.

Forts are not able to shield legions not manning forts, from archery or mortar-bombs if the assailant is sufficiently distant to shoot/catapult over the forts and strike the localepoints behind forts where the defender is trying to use the adjacent forts for cover.