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.