Kamianets-Podilskyi is an old town in western Ukraine that stands on a rock island inside a canyon. The Smotrych river loops almost the whole way around it and leaves a single neck of land, and everything built here answers that one fact. The pale stone underneath is the floor of a tropical sea more than 400 million years old, and in the canyon walls you can pick out fossil corals and shells. The river cut its ring through that limestone and gave the town a moat it never had to dig, which made it one of the strongest places in eastern Europe. A castle sits on the neck. The only road into it runs along an 88-metre span of stone, with the riverbed some 27 metres below. The Ottomans rebuilt it so thoroughly during the sieges that it is still called the Turkish Bridge. It is better walked than driven: the canyon falls away on both sides, and the gate towers grow with every step you take across.
Twelve towers once ringed the Old Castle and seven still stand, each raised or reinforced by a different century. You can climb them and walk the curtain walls between. One corner tower was rebuilt with money sent by Pope Julius II, though locals name it for Ustym Karmaliuk, the rebel who robbed Podillia's landlords and broke out of it in 1823. A footpath traces the gorge rim around the town, where the limestone falls as much as fifty metres to the water. That is where the shape makes sense: the complete ring of river, and the fortress on the one piece of ground that closes it. Other paths take you down to the bottom, beside the Smotrych and past ruined water mills. Down there the traffic disappears and the fortress is a silhouette. The Ottomans held this town from 1672 to 1699 and left a 36.5-metre minaret beside the Catholic cathedral. Nobody pulled it down. Since 1756 a gilded Virgin four and a half metres tall has stood on it, because the town turned the tower it inherited into the tallest pedestal it had.