At the top of the Scottish Highlands, not far from the UK's most northern point of John O'Groats, is this partially ruined castle. Built in the late 16th century by the 5th Earl of Caithness, George Sinclair, Keiss Castle was also used to defend nearby Caithness during the Second World War. Due to the positioning of the castle, perched perilously on the edge of the cliff, visiting the ruins is not possible. The views from afar, though, are stunning.