Unmonetised, open-access historical ruins, castles and ancient watchtowers. 17 so far.
One of the highest fortresses in the Apennines: a roofless 10th-century watchtower and ring walls on a bare limestone ridge at 1,460 m, free and open day and night.
The mud-brick remains of the medieval trade capital that funnelled Saharan gold to the Mediterranean, now a field of eroded walls outside Rissani that nobody guards.
One of the oldest fortresses in Georgia, a 10th-century keep on a rock at the meeting of two rivers, walked past by everyone on their way to Vardzia and entered by almost no one.
A 12th-century castle grown out of a sandstone outcrop on the Molina plateau, better known as the Tower of Joy than by its own name.
The red-brick shell of a 13th-century Livonian Order castle at the meeting of two rivers, its lower storeys drowned when the Daugava was dammed in 1965.
The concrete skeletons of a 1905 gold-rush city at the edge of Death Valley: a three-storey bank, a railway depot and a house built of bottles.
A roofless 13th-century Cistercian abbey in a cow field beneath the Rock of Cashel, one of Ireland's most visited sites, and almost nobody walks the 400 m to it.
The fractured remains of a 13th-century Welsh castle on a conical hill above Llangollen, open to the sky and the weather.
A roofless 19th-century mansion on the Aberdeenshire cliffs, said to have given Bram Stoker the setting for Dracula, and left to the gulls since 1925.
One of the largest castle ruins in Slovakia, a 4.5-hectare royal fortress on a volcanic hill that blew up in 1660 and has been overgrown ever since.
A vast Chandela-era hill fort on a flat-topped outcrop of the Vindhya range, with seven gates, a colossal rock-cut Shiva and a temple to the god who drank poison here.
The stone terraces of one of Japan's three great mountain castles, 717 m up in the Gifu hills above an Edo-era merchant street, free to walk at any hour.