You will reach a rather big area, where you will need to open another gate. Begin by approaching the main door to get the symbol from them. Then go down and go through the portal to reveal another hidden passage. Go there - enter the staircase and turn left. Continue towards the woods.
You will soon hear Valravn humming. You will also see him from afar. Approach him, and it will turn out it was just another trick of his. Continue forward, enter the wooden bridge and go further. There's Valravn again - not real, of course.
Go underneath the plank and unlock the gate. Then take the ladder up - Valravn makes one more of his amusing tricks. Go through the portal and you will see the tower coming back to shape. Go down, enter the portal and go towards the tower.
Take the stairs all the way up the tower. Look on the gate - combine the symbols so they create the desired shape, thus opening the door. Time to get closer to the lair of your deceptive enemy.