AI seems to pay poore care to Strategic Towns.
I noticed the "Confederate, AI player" is used to place just one militia unit to garrison most Strategic Towns, Richmond, the Confederate Capital too.
Also, the Union (human) player can quite easely occupy most of them: Norfolk, Charlestown and other coastline Strategic Towns can be occupied by amphibious assault carried by a one division force.
Forts are effective to protect major port-cities by blockade activity, but are uneffective against amphibious assaults: Attacker side can easely bypass forts, landing at any adjacent region, then moving to assault the target town. All that in the same game turn.
It's also true, AI seems not interested to re-capture enemy controlled Strategic Towns.
So once they have been occupied, player can use any week unit to garrison them.