Somewhere easy to wander into for people already in the world might be good like? IDK this is a school outing in a big Park in London middle schoolers learning BASIC RITUAL SUMMONING and it goes wrong like it does every year because GOD someone forgot the right virgin sacrifice--that's one option.

Another is to do it in an isolated spot like you suggest! DEEP BENEATH THE CHURCH and characters who don't have an IC reason to have joined in can wake up because they were brought there with the usual kosher method of being randomly kidnapped off the street. A strange labyrinth of horror you were kidnapped/summoned/stumbled into, something like that? Escape the horror terrors via sewer adventures could also be a thing!