system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] Why didn't Witcher Fortresses, such as Kaer Morhen, have a back-up trove for things like mutagen/alchemy recipes?

Surely, as much as Witchers plan for every scenario, and as hated as they are by 90%+ of the population, they'd have considered they might eventually be attacked on their own ground and prepared for such eventualities? Is this a simple case of Plot-Induced Stupidity (PIS)?

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As I understand it, peasants and some mages attacked Kaer Morhen. To attack a castle like Kaer Morhen, especially considering the inhabitants, you'd need serious numbers and preparation even if you did have sorcerers/sorceresses on side. How did they avoid any sort of detection via sight/sound/smell on the journey to the castle itself? 

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And if it is PIS then... well... *sigh*.
gpt
The mob of peasants outnumbered the Witchers by a huge margin and quantity has quality of its own. Doesn't matter how much stronger the average Witcher is over the strongest peasant, if you don't have the numbers to man the walls, a castle is pretty much a death trap. And the mob had mages supporting them. It's not a stretch to assume that the mages simply opened a portal for the mob to pass the Witchers' sentries.