Forts are a unit type that only has defense as it's stat. (This makes them untargetable by cards that require attack like raiding brigade or amphibious assault.)
They cannot move, and they cannot be converted, suppressed, or pinned. They may have keywords, and they can store a unit underneath them. The unit inside can still operate as normal unless otherwise specified. Forts can only store infantry unless otherwise specified.
Infantry inside cannot be targeted, though some effects may carry through. Suppression, pin, and retreat carry through when they would be applied to a bunker and affect the infantry inside. However convert, any damage, and direct destruction do not and will either do nothing or only affect the bunker. (Specifically convert does nothing. Direct destruction or trying to remove it from the battlefield do not work. 40. Panzergrenadier for example could do nothing against a bunker or the unit inside it. Units directly fighting via order or deployment deal their damage to the bunker first still. Excess damage dealt to the bunker is dealt to the unit inside instead.)
Keywords of units inside the bunker still apply. However, the bunker itself may have keywords that will override the unit. A unit inside a bunker with shock will be able to attack as if it has shock. The same applies to ambush or guard.
Keywords and abilities of bunkers only trigger if they have a unit inside of them. They cannot hold the frontline and will be captured if the enemy moves into it while they are empty.
Most fortifications will be neutral although they can be nation specific. They are obtained through orders, deployment effects, and maybe adding them in when you design your deck if they are nation-specific.
Units can be deployed directly into forts in your support line.
You can now move units around in your lines (like between two forts) as long as you pay the operation cost. Forts can only contain/store/have within up to one unit.