"Nothing we couldn't figure out, like hiding behind the door once it is unbarred, and then having either some meat on the floor to coax it out, or an illusion spell.
Or just gamble that I could outrun it and lead it to the cultists..?"
Sorry if this was misleading, the door only locks/unlocks from this side. As long as the door remains unlocked and unbarred you could open it from either side.
The key hole does not go all the way through, so you can't peep through. It is also fitted very well, so you would need to wild shape into something like a wood louse to crawl through any gaps.
"What if we unlock and unbar the door from this side, and got through the southern door. That way if we come across the creature and it is hostile, we have an escape route."
I'm not sure I follow that.. If we unbar the door, what stops the creature immediately coming out? And how likely are we to get trapped in there, knowing that it exists?