Occupying Trial Bay's eastern headland, this sandstone prison was built between 1877 and 1886 to house convicts brought in to build a breakwater. When nature intervened and the breakwater washed away, the imposing structure fell into disuse, aside from a brief, rather tragic, interlude in WWI when men of German and Austrian heritage were interned. Today it contains a museum devoted to its unusual history; even if you don't visit within, it's worth a detour for the views and the resident roos.
It's a pleasant 4km dawdle along the beach from South West Rocks.