Topic: Crimson Shroud help

Hello all,
I'm stuck in Crimson Shroud at Chapter 3... I guess that a lot have this game because it's great.
So, can anyone help me out? I am on a map called RAHAB Palace Proper and I found an item called Serpent's Key which I think is essential and I don't know how to continue from here on...

It's been awhile since I've last played, so my memory is a little foggy, but I think I remember this...

Go back to the underground tunnel (where you were in chapter 2), and there should be a gate you couldn't pass before, but with that key you can.

Thanks but how can I get back?

EDIT: Nevermind thank you