Where should I go from here?

Hint 1 of 10:

Isn't that obvious?

Hint 2 of 10:

If you opened all doors, you've at least tasted the flavour of the next area
to go to.

Hint 3 of 10:

You probably haven't been there yet and you already discovered two, or maybe
even three ways to enter it.

Hint 4 of 10:

Yes, you should certainly pay a visit to the mines.

Hint 5 of 10:

Don't enter them from here, however.

Hint 6 of 10:

Leave the planetarium room through the hole in the floor (you got here this
way, remember), but don't forget to open door number one first --  you'll
soon find out why.

Hint 7 of 10:

You'll end up in the garden again.