While looking for examples of fortresses, I found this page...
gauer - Game Collections
...Google ranked the linked pages ii, iii, vi, vii, and viii high on a search for 'fortress', while at least one other page has one as well. The games I looked at were middlegame positions like blocked Pawn chains rather than endgame fortresses, but I don't want to be too narrow-minded about the term.
The introduction to the collection starts, 'I have attempted to provide a few of the strategic axioms of chess (il)logic.' That's a novel way of classifying chess positions.