Cool story!

I never thought about the Fortress key/marker, but it makes a lot of sense that something like that would have been critical to early air travel.

"As for contentment, that is a myth.  Give a man everything he wishes and he will be unhappy, because he didn't wish for more; give him more, and he will die of his worries.  Only a turtle, asleep on a sunny log, knows contentment!"
-Merlin to Prince Valiant

            Hal Foster 3-7-43