It's too obvious. But kinda not. I'm thinking Amy isn't Amy, she's one of the monsters. Harper may or may not be crazy, but she's definitely aware of these things.

I'm thinking they need something like a guardian to willingly give up their post.

Prison makes it impossible for them to leave unless the guard willingly succumbs or cedes their ground. Amy is probably just as trapped, maybe her malevolence is inside one of the off limits paintings kept on the upper floors.

An "unwilling warden," if you will.

I'm definitely looking forward to what happens next