How can I light up the Underground?

Hint 1 of 7:

Did you try your lantern?

Hint 2 of 7:

Hmm...well, perhaps you could "let the sun shine in".

Hint 3 of 7:

Have you seen anything that could reflect well on your chances?

Hint 4 of 7:

Don't continue on until you have visited the Royal Castle.