The purpose of saying "a fortress is breached from the inside" is to tell everyone: Be wary of the enemy agents within us! Take precautions before they happen (to prevent unexpected events). There is also a second meaning: If we want to defeat the enemy, it is best to send our experts to sneak into the enemy first, and do a "cooperation from inside to outside", and it will be much easier to destroy the enemy!

Fortress is synonymous with "iron wall".

"The fortress is breached from the inside" means: if there is no problem with the external defense, then there is a problem internally. "Self-examination" is required.

A fortress is like a golden mask, which can refer to a solid object or a tightly knit group. According to dialectics, external factors work through internal factors. In other words, it is easiest to break through only through the inside.