"Large Troops Entrenched vs Forts, how to handle?" Topic

Okay guys, been doing a lot of back and forth on this for my board game(labor of love), strategic level. We all know there were "forts" and there were large groups of troops that were in heavily fortified areas, just as hard to take, but much larger than the "forts". Examples that come to mine are Vicksburg & Petersburg.
So, my "problem" is how to show the difference on the game board of a group of troops much, much larger than the forts but just as hard to take and be fair and balanced about it? Vicksburg comes to mind the most.
My first thoughts are if you have an army and a fort in the same area you must first drive the army out before you can attack the fort. Also these forts, depending on size can only be reinforced with a limited number of extra troops.
My thoughts to represent, say Vicksburg was that resources and game turns had to be spent to build up a large area that could hold basically an army but be almost or just as tough to take as a fort.
But what if the army than later moves away, should all the protection in that are go as well, if not a city? After all since the areas represent large sections of a state even if the large section of rifle pits, extra supplies, etc stayed, most likely another battle in an area that large would not be fought in the same spot or from the same direction.
So, do you guys think I have a reasonable compromise to do this situation justice but keep it as simple as possible or do you guys think there is a better, more "historical" way to handle it? Thanks in advance for any suggestions:) Happy gaming all!

Permanent forts and entrenchments are one thing, lines of extensive field works are another. And whether or not troops occupied or were outside of these depended on the situation. The board game, "A House Divided," which is strategic level, handles this by allowing some of the chits in a given area (movement zone) to be covered by a fortification marker while others are not, although this really represents more permanent lines of entrenchments, not temporary field fortifications. The difference is that a movement impulse must be spent for the former to dig in, while those who just arrived might not be so covered. Then there is the question of whether the troops outside the trenches are helping to defend the city or whatever, or are a relief force sent to try to lift the siege. It's largely a matter of scale and what one is trying to represent. The possibilities are endless. During the American Civil War, a lot of the entrenched lines, especially around the industrial centers like Richmond and Atlanta, were pre-constructed using slave labor. The Confederate troops simply occupied them when they arrived, and then worked to improve them while the siege was going on. I'm not sure about Vicksburg, but my impression is that most of the works there were built rather hurriedly by the troops of Gen. Pemberton's army as Grant's army was closing in. After capturing the city, most of the Union army remained there for several months, during which time they pulled down the earthworks so that the Rebels couldn't re-occupy and use them later. The earthworks one sees at the national park there today are actually later 1930's WPA reconstructions. The fortifications around Vicksburg covered a relatively small area around a port city. They could easily have been reoccupied and used if the Confederates had ever recaptured the place. Those in the Wilderness, Spotsylvania and the various parts of the Atlanta Campaign were more temporary field fortifications, not nearly as elaborate, and probably would not have been of use again unless the two armies returned to exactly the same lines, which would have been unlikely.

Thanks for the input, good points all, BTW, just FYI I actually live in Vicksburg and you are correct:)
Yah I have seen several games that has a system that allows for cities to have defenses improved but if one side is defending or next to a fort they just get some type of extra bonus. I think for that level game that is pretty close and practical but just a little to generic for my taste.
From your input I think I am on the right track allowing a particular objective(city) to be built up with time and resources. For a "meeting" engagement the defender will only have the option to build limited works, based on leadership, and then possibly have the option to improve them to "one level higher" if the battle last more than one day…….??? But like you said after the battle, unless its a particular objective, like a city, as soon as the armies move off the defenses are considered to no longer be there for practical reasons. Also have the year of the war have an influence on how likely it would be a defender can dig in since, as we all know, the longer the war went on and tactics changed, lessons were learned, the more comfortable each side got with the idea of digging in and the better each side got at it. Thanks again for the input! Happy gaming.