Ice tower

Hai!
How can i build the ice tower? I'm now at the Wulf Island, but i have the feeling i missed something, because if i want to open the treaser(?)chest they asked for a ice tower... Please help me???

Sorry... I got it;-) My englisch is not that great, but i did read the Walkthroughs...
So your help is no longer needed...

hallo, wie schalte ich die große oase frei...........bzw. die verlassene stadt?