cave prison; cliffside jail
Rare term for a prison or jail built into a cave or cliffside, often in historical or fantasy contexts.
その物語では、反逆者は岩牢に閉じ込められた。
In that story, the rebels were confined in a cave prison.
Compound of 岩 (iwa, 'rock') and 牢 (rō, 'prison'). The exact origin is uncertain; likely a coined term for a prison in a rocky location.