Every castle is undefeated… for some while!
Literally: difficult – attack – not – fall
Alternately: Nigh impervious to assault. Difficult even to attack, much less to actually bring down, like a near-impregnable castle. By extension, a situation where things don’t go as planned, expected, or desired.
Notes: Sources suggest that this compound was formed in the Sengoku period, and note wryly that few of the era’s supposedly-invincible castles remained undefeated.