This one I really don't get. 
1. It's a huge aid to those recovering from wars
- When a country's army gets destroyed they have to rebuild it somehow, and the only way it can be rebuilt is one regiment at a time. This makes it very easy for attacking countries to simply destroy it again, wasting the already weakened defender's manpower and money. A country in this situation needs to have a tool they can use to preserve their small armies, and to give them some sort of use. In this game I often see situations where larger enemy army will travel across their enemy's entire country to seek out a weaker one because they know they can defeat it - this is absurd because hunting down and forcing another army to fight was far from simple in this era. One way of dealing with this could be to add a "scatter" button to an army, whereby it'll automatically split up into smaller groups and garrison in the closest nearby fortresses.
2. It gives smaller countries a better defense.
- Likewise, it gives the armies of smaller countries an option aside from just being wiped out for no gain, again at a loss of money and manpower. When the Ottomans attack the Byzantines and their army is approaching Constantinople, the Byzantine army should have some choice other than simply standing there and dying. Let them enter the fort, but with a limit - only 2x the fort level, so the Byzantines could put 6,000 extra men in their level 3 fortress. This would also allow fortresses a variable number of defenders, which makes it more historical. A feature like this would also enable the defender in a war to tie down a larger number of enemy troops in a siege.
3. We have a sortie feature now.
- Now that we can actually use armies in fortresses to engage in combat, there's a real use to having armies be stationed there. We can tie down large numbers of enemy troops and prevent assaults by selectively garrisoning our forts, and sortie from them using decent sized armies that don't just get wiped out immediately.
So come on Paradox, let's do this already.
1. It's a huge aid to those recovering from wars
- When a country's army gets destroyed they have to rebuild it somehow, and the only way it can be rebuilt is one regiment at a time. This makes it very easy for attacking countries to simply destroy it again, wasting the already weakened defender's manpower and money. A country in this situation needs to have a tool they can use to preserve their small armies, and to give them some sort of use. In this game I often see situations where larger enemy army will travel across their enemy's entire country to seek out a weaker one because they know they can defeat it - this is absurd because hunting down and forcing another army to fight was far from simple in this era. One way of dealing with this could be to add a "scatter" button to an army, whereby it'll automatically split up into smaller groups and garrison in the closest nearby fortresses.
2. It gives smaller countries a better defense.
- Likewise, it gives the armies of smaller countries an option aside from just being wiped out for no gain, again at a loss of money and manpower. When the Ottomans attack the Byzantines and their army is approaching Constantinople, the Byzantine army should have some choice other than simply standing there and dying. Let them enter the fort, but with a limit - only 2x the fort level, so the Byzantines could put 6,000 extra men in their level 3 fortress. This would also allow fortresses a variable number of defenders, which makes it more historical. A feature like this would also enable the defender in a war to tie down a larger number of enemy troops in a siege.
3. We have a sortie feature now.
- Now that we can actually use armies in fortresses to engage in combat, there's a real use to having armies be stationed there. We can tie down large numbers of enemy troops and prevent assaults by selectively garrisoning our forts, and sortie from them using decent sized armies that don't just get wiped out immediately.
So come on Paradox, let's do this already.