This position from Boland's Bridges caught my attention because of its practicality. Black has sacrificed a man in order to break through for a king while maintaining a strong defense with his remaining forces. The caption "White to Win" should bring hope since many of us have faced an uphill battle when caught between a fortress and a marauding king. In this instance, at least, the aggravator is punished for this cheeky behavior.
There are three strategic possibilities for White:
(1) Allow the man on square 18 to crown and attack White's pieces from behind. The tradeoff must be that White gains a move that breaks through Black's fortress without losing material.
(2) Move the piece on square 17 in the hope of having more waiting moves than Black.
(3) Move the piece on square 17 with the intent of sacrificing a man through square 6 in order to crown and attack Black from behind.
White's first move is an important point that should clarify how to both defend and attack this type of fortress.White to Win.