Fields of Despair - GMT P500.
My understanding is that an enemy fort still acts as an enemy unit, in terms of movement restrictions. Though you have pulled back your mobile military units, there remains a crew manning the fort. When would your artillery be unable to damage a fort? Forts take a hit on the initial "6" (see exceptions for Big Bertha), and then on any remaining "6" if there are no remaining infantry/cavalry SP's.
This is correct.