If you are at the higher side of the first stage(where you can reach after you climb up the rope), you mean you found 'A key for somewhere' and ulocked the portcullis, ran through the portcullis and you stuck at the end.
You need to hide inside the room where you can find as soon as you enter the portcullis and wait until the orc bypasses and unlocks the iron lattice door(its actually not unlockable for player, only orcs can do).
Don't forget to close the door when you are hiding. Orcs gonna check your room if it's open.
If this is not your case, please inform me.