noun
solitary castle; isolated castle
Refers to a castle standing alone, often in a remote or besieged position. Can carry a literary or historical tone.
その孤城は山の上にぽつんと建っていた。
The solitary castle stood alone on top of the mountain.
敵に囲まれた孤城で、兵士たちは必死に戦った。
In the isolated castle surrounded by enemies, the soldiers fought desperately.
Compound of 孤 (ko, 'solitary') and 城 (jō, 'castle'). The irregular form 弧城 uses 弧 (arc) instead of 孤, likely a phonetic substitution.