Mousa Broch
The island of Mousa, just off the east coast of the Shetlands, has the most complete broch in the world - at least, externally. (There are one or two others where the inner arrangements are better preserved.) You can climb to the full height of the broch and stand on narrow walkway that circles round it but be warned, the ancient builders arranged their steps for their own convenience, not yours! I fell twice in the dark lower portion and normally I'm pretty sure-footed!