Or give the spiders some levels in barbarian.
Perhaps not the spiders themselves but ettecarps definitely. Monsters with class levels may be the key.
Throw in some drow. Drow and spiders just naturally go together. Maybe even a Drider.
Drow themselves don't mix thematically here (as I said, there is already a "creature" controlling the spiders and she would attack drow as readily as humans - she is a vain woman turned into a bloated monstrocity and kills any humanoid creatures for seeing her like that. But since she already controls the spiders, I can give her any number of related powers. If I make her a sorcerer, she won't be easily interrupted with her current constitution score.)
Or maybe some Treants or nymphs or dryads who use the spiders to keep people out.
Two druids in the party. They would just ask nicely
The idea is shaping up. Thank you all for your contributions.