In the woods, you surely know the place where you have the tower on your right and a smaller lake on your left. The solution is easy: You have to turn left and get past the lake, you will find a small path that actually takes you up to the tower.
Okay, so here's the detailed version:
When you go through the tunnel that connects Vergen to the woods and the crypt, you arrive at the big lake. Directly in front of you is the hill that you have to take:
There's only one way to cross the lake, I'm pretty sure you figured that out by now. :) When you arrive at the intersection, take the left path.
On your left, you'll to see the small lake:
Take these directions:
You will see this; go all the way down on the road…
… and you will arrive at the tower.
I think that's detailed enough. :)