Narenj Castle is a ruined mud-brick fortress on a mound in the old town. Its origins are usually described as pre-Islamic, and although much has collapsed, the surviving walls still give Na'in a deeper time horizon than the mosque alone.
The castle is best seen as part of a walk through the historic quarter. Do not expect a restored fort; the appeal is the raw silhouette and the sense of a much older settlement underneath the present town.