The reaction is all but immediate, and suddenly Dean wonders if this person came here searching for his prisoner. It makes sense. He captured someone to keep them from running off and sharing his location with an angry village, and he still gets invaders in his castle.

He follows quietly enough behind.

"Castiel? Is that you?" The voice is undoubtedly Castiel's father, trembling from cold and fear and sudden hope. "Help me. Find the key." And his hand reaches through the bars of the dungeon he's in, hoping the movement will catch Castiel's eyes and lead him to where he's being imprisoned. "Hurry."