The short question is: Can a large creature (such as a Troll or a Drider) pass through the narrow passage?
The official answer is apparently a topic for debate (that I want to avoid). For example, this post opens a can of worms and proposes houserules while this short forum post seems to briefly conclude that a troll can, in fact, pass through the narrow passage.
I'd normally just propose a ruling stating that trolls and driders are indeed able to pass through. The odd thing is that Errtu's card specifically says:
can be placed over walls, but the center square that this figure occupies must be open.
We don't have a similar clarification for trolls and driders.
Would the lack of such clarification imply that a large creature can't pass through a narrow passage?