How about this suggestion?
Each and every fort can have their garrison size upgraded by a certain percentage using money. This increases the defense efficiency of the fort and the number of men needed to siege it. However, there are some draw backs to this suggestion. The higher level the garrison, the higher maintenance even while mothballing. In addition, there will be an upgrade deficiency to slowly increase the cost of the upgrade.
An example:
Constantinople can have their fort tweaked down to level 1.
Starting from that level 1 fort, we can make some serious adjustments to it, but first lets add some upgrading rules.
a) Every garrison upgrade increases the fort size by 500.
b) Every 2 garrison upgrades increases the number of troops needed to siege it by 1.
c) Every garrison upgrade increases the fort defense efficiency by 5%.
Now let's rename the fort to Fort 1A. The farther down the alphabet, the more upgraded the garrisons are.
Constantinople gets their fort upgrade to Fort 1K. That is a total of ten upgrades. I would say it is reasonable for the upgrades to cost around 3000 ducts, because the Byzantines were very rich before their decline. In other words, the first upgrade would cost around 100 ducats and the last upgrade would cost to somewhere along 300 ducats. A class J fort has the following statistics:
-2000(base)+10,000 soldiers
-3 regiments to siege + 15 regiments
-x% of defense efficiency + 50%
Close to historical right? Let's try to figure out the maintenance cost. Each upgrade increases the total maintenance cost by (level of fort=1)*(1+0.5x). This would total 6 ducts to maintain; however, Constantinople is the Byzantines' capital so it would have no maintenance cost.
I'd say this is a fairly historical attempt at trying to balance forts. However the real problem is the army size + manpower. If only army sizes could be historically increased based on region. The Farther east you go, the higher the manpower + army size. For example, Ming had around 500K or more men during the 1400s. The Ottomans had around 100K+ men and France had only around 50K most during the 1400s.