I wonder what this door was made for

Access for that hard lid ceiling

The monsters sleep up there.

Narnia

The world may never know......
Until someone up there walks out without knowing

Possibly, that is where the ghosts live.

ninja class, only those with the skills may eneter... as you can see the line outside

The ultamite cliffhanger challenge

parallel universe

My guess is, at one point, they were planning to put a staircase there, and got the door but then stopped with the plans.

That door is meant to keep people out... or more disturbing, to keep something IN. So I think it's best to leave it shut, mkay?