armchair athlete, I'm quite aware of letting the AI capture it but since I have to take it back, it often gets knocked down two levels due to the siege from the AI taking it and you taking it back.
I don't like the AI to take my provinces anyhow.
BTW, what you observe is the structres going down levels.
the castle upgrades also count so the fortress (plain one) was downgraded to citadel and demiculv towers.
anyhow, the citadel in big C, the only high level fortification in many cases, start as having a barbican upgrade so it's rarely reduced to castle and thus people don't notice.
the keeps however, frequently become fort, motee, and bailey.
actually the higher level castles downgrade pretty often also.
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