To get to the knight, enter the cathedral from the "teleporter" (and have the doors open, meaning the two statues are placed in level 3, not level 1), this should unlock the bars. Anyways, from the teleporter, go up then in the new corridor go LEFT. The right would take you to the part with the boss and the bomb (but you need to unlock the door from the other side first). Anyway, once you take a left it's a straight path (as in, there are no side rooms or rooms that will lead you anywhere else, just continue exploring) to the room with the knight (if you've done simon's quest you'll have seen him from the other side).