How do I get into a specific part of Hyrule Castle?

I’m spending my afternoon playing Zelda: Link to the Past on my SuperNES. It’s always irritated me a bit that I couldn’t figure this part out. In the dark world when you start out on Hyrule, there’s a hole in the wall that’s barricaed by two statues. Superbomb is a no-go. What do I do?
While I’m at it, there’s a ledge in the mountain range near Turtle Rock. In one world there’s a opening, in another there’s not. I’m supposed to drop down and use the mirror, I think.