A tunnel
After they investigated the creature further, they kept their distance from it. They didn’t want to get close at all, they had no idea if it would wake up and how secure those chains were. Maybe it would have enough slack on the chain to move around the building and attack them.
Just when they thought they had seen enough though, it was just getting started. Right behind the creature, there was a door that was leading to some other room. But that didn’t make any sense because the structure was just four walls from the outside. They knew that they needed to go through that door.