As he said, spiders can climb walls, but if it's completely surrounded in walls, with sufficient light, nothing should be able to spawn inside, and unless they jump off cliffs near your house they shouldn't get in.
I sort of like the idea of enhanced, rare monsters to get special items, but some of them just seem stupid, making it harder to hit to get more of the same thing, when you could easily kill more of the easy target and get as much stuff.
I have a terrible computer and this has never happened, probably a mod or something you put into it. Try a fresh new download and only put in the most important mods, none if you can help it, and test that.
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