Viking: Battle for Asgard

You have to go around the east side of the fort, look for a flag with a picture of a tree on it, you can jump that part of the fence. The rest is just stealth, I went to the right and followed to wall pretty much but there might be other ways. Watch out for the hornblower as that spells certain doom.

I also climbed over the flag on the east side of Darkwater, but I turned left and made my way through here.
You have to kill some enemys from behind, so you should have learned the move Baldur' s Silence from the arena!

Go on, until you reach some kind of red glowing flames. Here you can recharge the amulet.