I've cleared out everything at both levels in the Ice Temple, and the only thing left is opening that shadow door and finish off the Caged Fury. But I can't open the door with the prism device, and Nicademus has no help to offer me. Is there something important I haven't done yet?

Well you need to change the direction of a mirror not far from there, so that it faces left/east. then pull the both levers (sorry don't remember which lever first). A lightning will fire away and the door will be open. That's it!! Good Luck :)

I've followed the walkthrough on that point, and turned the mirror left before firing at it. It doesn't open the door.

Which door? There is a door right on the hallway, and there is another door that leads to where Caged Fury is behind that hallway door.

The hallway door you can open from the other side by choosing the right words on that magical stairway on the first level (I don't remember which words transport you behind the door though). The inner door is opened by the lightning machine.

Are you sure you are about to open the shadow door? The shadow door has literally a shadow on it. Before the shadow door there is another locked door which you have to get pass it using the talking picture upstairs.

While waiting for replies, I finally found out what to do :p . I was talking about the "hidden" door which says "EMBRACE THE SHADOW" when you try to open it (the first door, which leads from the hallway to the room with the shadow trap). I found out that the painting could teleport me inside, and though that door was still difficult to open (for some strange reason), a lot of clicking on it got me out :) .

That's what the other guys have tried to tell you :p well, anyway good job ;)

Once inside the "Embrace the Shadow" room, the way out requires one character stand on the shadow while another one opens the door. You probably managed to open it accidentally because one of your characters was on the shadow. Once that door is open, the lightning bolt opens the inner door.