Hidden in the ruins on the left side of the land, after you built the stone
bridge to cross the river. Across the river is one set of ruins, then to the
left and south of that is a large briar patch that needs to be removed. Once
the briar patch is removed you need to cut down the large dead tree to get 
across the chasm to the last area. There in that last area are the ruins of 
the observatory. Once your workers clear that area the Libra Key will appear.