Help! Stuck in "The Swiped Signet" Dungeon...

[Jemaniya]

nvm

[pow_lee]

I guess you are missing the levers on the platform the elven archers are on. There is valve iirc and the door should open by using that.
After passing by the locked door, set back on a kind of ledge the planks go on and one path goes to the mentioned platform with the elven archers.

The whole spider business is behind the hidden door and the quest proceeds from that point on, after you pulled the switch in the main room where the spiders are (far left corner)