7 Forts

1-Boring

When this fort was first built, there was a serious danger from the warlike tribes of the area, but they were either defeated or pacified a century ago, and the overwhelming foe here is boredom. It could have been closed decades ago, but it is kept open as an unofficial place of minor punishment for failed officers and men, who have not done anything bad enough to justify time in military prison, but have messed up in some minor way. Those who have done something bad but it can't be proved, or the military want it kept under wraps rather then bringing it to a court-martial, are also sent here. Whilst it is a safe place to be, it is incredibly boring with nothing much to do. If in the modern day, no Internet or phones are allowed, officially for Operational Security but mostly to stop the soldiers who are sent there from avoiding boredom.

Real Life Example-Certain forts in the later Wild West era.

2-Forward Operating Base

Based somewhere like The Field of 10,000 Skulls , this place is excessively dangerous with battles and raids happening almost every single day, and being sent here for too long would be a near certain death sentence, so military units are rotated in and out every month or so. Boredom is not really an issue here, replaced with the adrenaline and fear of death. It is possible here to earn promotions or your world's equivalent of the Medal of Honor here, but it is also quite easy to end up in a military cemetery or being wounded badly enough to be permanently invalided out of the Army. Even soldiers who look forward to engaging in combat, don't want to be sent here for too long. No outside patrols take place, as leaving the base for any long amount of time would get the soldiers either killed or captured.

Real Life Example-Bakhmut in Ukraine at the time of writing of this submission.

3-Keeping Others In, Not Out

The walls here are not about keeping foes out but about keeping prisoners in-prisoners that have either committed horrifying crimes or would be a threat to national security if they escaped. Some of them are members of criminal and/or terrorist organizations who could set them free if it were not for the high security, others know too much about the wrong thing. Most of the prisoners here will die in captivity, and the troops here are more de facto prison guards then soldiers. In some cases the prisoners de facto rule inside, like the H-Blocks of Northern Ireland before the Good Friday Agreement, whilst in other cases the guards rule with an iron fist like Guantanamo Bay and torture the prisoners.

Real Life Example-Florence ADX.

4-Point of Contact

Centuries ago there was a war with otherworldly people, elves or orcs or tentacular-monsters, and they were driven back to the orginal place that they erupted from. Since then, relations have thawed out a little bit, and both sides have things to trade, but all the trading is done at this one fort that encircles the sole link to the other world, as there is no way at the moment that the otherworldly race is going to be allowed to enter the world again after what happened the last time.

5-Here be Treasure

In this case the fort protects the gold reserves of the country from being stolen by the criminal underworld or anybody else. Protected by all kinds of security measures both open and secret, is a fortune of treasure. If the place could be successfully raided, the robbers would be rich for the rest of their lives, but it is thought to be impossible to raid it successfully. Perhaps the PCs are attempting the robbery of the millenium.

Real Life Example-Fort Knox.

6-Cadet School

Once an active fort in a warzone when it was first built, it has been centuries since a cannon ball last crashed against it's walls. The place is now the top military academy/military cadet school of the country, and those lucky few who can get a place and avoid getting expelled for breaches of the strict rules, are sure to have a glittering military carrer in front of them. Former cadets help each other get promotions and cover up each other's mistakes when necessary. 

Real Life Example-West Point

7-Occupation Zone

Most of the time, this fort is safe, but there is an active resistance movement that hates the occupation and will happily kill the soldiers and attack the fort itself if they have a good chance of getting away with it. Here the danger comes from snipers and RPGs, or being shot or stabbed or blown up when out on patrol. Attacks on the fort itself are rare but not unheard of.

Real Life Example-Northern Ireland