Theodor must've told you about the door. It is shut down and can be only opened by solving the clues.

Yes. Note that you must have talked with Theodore about the clues before Anna will find them.

Now, onwards and find out the clues!

Those two big windows look interesting.

Look at the right window. Anna will spot some peculiar symbols in the corner.

Wouldn't it be nicer if the symbols were more light up?

Or the whole window would have more light. Go to the back alley to find the lamp post.

Light it up.

What is it lighting?

Yes. But also the church organ.

You would just have to know what keys to play.