Yes the fortress is very difficult to achieve. Storm has managed it, and in a game against George Miller I attempted it but failed to see a capture threat, he took my piece leaving me with only 3 spare white pieces :(
(hide) If you want to play a game with an opponent of a similar level, you can define a required BKR range for a new game invitation. Then nobody with a BKR outside this range will be able to see/accept it. (Katechka) (show all tips)