When are forts worth it?

[Satoshi]

It seems like they're a massive expense on the treasury, I usually destroy many of them unless its a particularly valuable choke point. When should I be keeping them/building new ones?

[jomigaru]

They reduce unrest. They may dissuade a belicose neighbour if the border both of you share has some forts.
I find them necessary but they must be careful placed.

[Menkerot]

useful for guarding the border with a powerful country. Other than that - only in capital in case of a civil war.

[galadon3]

You want at least one per province to avoid the enemy being able to just walk with a tiny army into the province capital and seize the whole capital.

Also you have a fortress-capacity, fortresses get really costly if you get past that cap. For more details:
https://imperator.paradoxwikis.com/Infrastructure#Forts

[Jean-Maurice Nya]

Past a point, I only have big level 3-4 forts on my borders where armies have to go through specific provinces and cannot go around it. Otherwise my armies will take care of pushing back AI armies.
At the beginning, fort on every province, on difficult terrain if any. And level up to 4-5-6 on borders depending on my finance and ennemies.

[Feynrik]

I personally do not care about forts so I dread the Armenian fort achievement to get that level max fort thingy.

[Jean-Maurice Nya]

It's not even level max. You can go higher than level 15. I don't see the point to go above 5-6 as AI is bad at dealing with food reserve and fort assault.

[Lateralus]

I place at least one on every province capital to make capture difficult, or on a boarder choke point if relevant. But the maintenance costs get expensive so I remove most of the AI built ones after I've captured a new province.
They also help counter territory unrest, but the returns are diminishing after the first fort level, so happiness (conversion, integration or assimilation) is a better counter for unrest in the long term.

[Aslan Ponto]

Provinces that are likely to become a Casus Belli war goal and are somewhat far away from the core of your realm. You might be busy elsewhere and not able to send your armies to that front for awhile, so that province must be able to resist on its own. Especially important if that fort can block the way towards more of your provinces. Now, the danger is if the enemy manages to capture it, you would have to recapture that high level fort. Whatever it suits your style.

They might also be very useful for minor powers with little manpower but good economy. Each level fort increases the garrison and the fortifications, so you are having "soldiers for free". If it is high enough, only a massive army can lay siege to the city, and the more troops gathered, the higher the attition. They might be even forced to leave the siege. Alongside Frontiers provincial law and some other bonuses, some nations can take advantage of a defensive strategy.

I agree that once you are somewhat powerful they make less sense.