금성철벽金城鐵壁 (쇠 금, 재 성, 쇠 철, 벽 벽)
geumseongcheolbyeok
ENa castle 성 of metal (쇠 금) and walls 벽 of metal (쇠 철). As to what kind of metal is 쇠, however, I want to say steel, but they didn`t have steel when they came up with this expression, so it`s probably iron then