He thought about it as he gave his chin a thoughtful stroke. "You'd need a key. I had a mind to keep them locked up tight."

Gendry had heard Sansa's concerns about the fairy ring and so he reckoned that such a doorway should be kept secure. Certainly if Sansa was concerned, the dwarves might have their own concerns at some shardholder maintaining a portal into their city. It wasn't safe.