Castles: storming vs besieging

[Gilrandir]

I have noticed that when you storm a castle the very building survives undamaged, but when you sit and wait for the garrison to surrender the outcome is doubtful: sometimes (though rarely) the building stays intact, in other cases (in about 90% of them) it is downgraded one level (e.g. if it was a keep, it becomes a fort). First of all, it doesn't seem logical as in real life storming presupposes some degree of ruination while besieging does not. Second of all, I would like to know how can one be sure that when the siege is successfully over you will "keep the keep" without downgrading. Any tips?

[Prince Cobra]

I have noticed that when you storm a castle the very building survives undamaged, but when you sit and wait for the garrison to surrender the outcome is doubtful: sometimes (though rarely) the building stays intact, in other cases (in about 90% of them) it is downgraded one level (e.g. if it was a keep, it becomes a fort). First of all, it doesn't seem logical as in real life storming presupposes some degree of ruination while besieging does not. Second of all, I would like to know how can one be sure that when the siege is successfully over you will "keep the keep" without downgrading. Any tips?

I don't think you can be sure of how much the castle will downgrade.

When storming you also don't have a guarantee that it won't downgrade but you will lose less buildings. The last 3 times I stromed a fort, it downgraded to dust each time. It may be the fact I attacked the fort from at least three directions (the same for the other more improved defences when I have siege equipment.)

P.S. In relation with the other thread, spies, if they have a high chance for success are a good alternative.

[I of the Storm]

My current working theory, yet to be falsified, is that it depends on whether extensions to the fortification have been built or not. Each time a province is taken, the fortification is diminished by 1. If you have a fort with motte and bailey and it's taken, it will be a fort with motte. If you have a plain fort, it will be gone once the province is taken. (this corresponds with the fact that fort level counts as "castle" internally while fort +m+b counts as "castle2" IIRC)
I think there was a thread containing the correct maths of this a while ago.
So what it boils down to is: build extensions to each fortification.

[caravel]

That is correct. The "upgrades" are not treated as such by the game engine, but as additional castle levels (which is what they are in fact).

[gollum]

+1 to I of the Storm's and Asai's point; these are my observations as well.

[Gilrandir]

So what it boils down to is: build extensions to each fortification.

But you storm your enemy castles, how can you build extentions to somebody else's castles?

[I of the Storm]

That should read: "build extensions to each fortification of yours - just in case..."
You can't help the AI obviously, seemingly it hardly builds extensions.

[drone]

The castle extensions can be beneficial to virtues (cheap and fast for the Builder line), but I usually don't build them if the province is in a contested area. They don't add food/capacity to your besieged units, and if you lose the province you have now gifted the opponent with a higher level fortress you will have to retake later.

[Ironside]

Second of all, I would like to know how can one be sure that when the siege is successfully over you will "keep the keep" without downgrading. Any tips?

Bribes. Bribing the castle garnison will change the castle to your hands, leaving every building intact. Not always easy to pull off though.