Romaji: nankou furaku
Literal Meaning: Difficult to attack, won’t fall.
The entire sentence here is 今やあの要塞は難攻不落だ。
要塞 (ようさい) means fortress.
今や well, 今 (いま) means now. The extra particle や is for emphasis, and a smaller nuance that I won’t get into here.
So here Claude is saying, “Now that fortress is impenetrable.” Tough to attack, impossible to topple.
Initial searches for this term online mostly resulted in websites explaining what the term means… I did find this Japanese manga that has the term right in the title though. It’s called 難攻不落の魔王城へようこそ and it’s on Amazon.jp.
Fire Emblem Three Houses is FILLED with advanced Japanese phrases and vocabulary. The game is entirely voiced, and you can replay any dialog you want as long as you don’t leave the dialog sequence. The Nintendo Switch is region free, and many (not all) games, including Fire Emblem, have the full Japanese text and audio available in the U.S. release of the game. No need to import from Japan! This is an amazing resource for gamers who are learning Japanese. Look forward to more. This game is the gift that keeps on giving.