「不落」は単独ではあまり使われず、難攻不落の一部として見かける。
不落 is rarely used alone; it is mostly seen as part of 難攻不落.
impregnable; unassailable
From 不 (not) + 落 (fall). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning 'not falling' or 'unconquerable'.