there is a spiral staircase in the temple with yorda in the back that leads to a door. Where does it go?

It leads to the giant bridge that you crossed to get here at the beginning of the game. You can actually make it to that bridge and beyond to the secret garden if you climb the temple.

And it is actually Mono, not Yorda. That's my fault. In your bird thread, I refered to her as Yorda (Yorda is the name of the female character in the game ICO, the "pre-sequel" to this game).

thanks