The top of the hill was a natural fortress.
That was dangerous, hostile country, a natural fortress, and no one from the Federation wanted much to do with it.
It is bordered on one side by the town's natural fortress, a high cliff.
The island was a natural fortress further improved by human ingenuity.
The new settlement was very fertile and had a natural fortress.
They maintained this natural fortress against many fierce attacks and during the whole of a winter.
The site they had chosen was protected by a natural fortress of pale pink rock.
In the course of history, this region was a natural fortress that invaders found very difficult to conquer.
Malan's men had already sought shelter in the same natural fortress.
Mallory understood him: it was a natural fortress, with a little work.
