"Where does it take you if you go through those trees" the couple asked me as I left the main track. When I informed them it took you to Rose Castle Tarn, they said "No, it's called Tarn Hows, and we're walking all the way around it". Despite my best efforts I failed to persuade them that there really are two tarns here. Such is the secrecy of Rose Castle Tarn, unmarked on the map and seemingly joined onto Tarn Hows itself.
But the tarn is indeed independent from Tarn Hows and held in place by a short dam. The tarn is attractive enough in the right conditions, however the most interesting item you're likely to find is the 'hut' which overlooks the tarn.