Ninh Binh
The Trang An river winding through limestone karsts, Ninh Binh. Photo via Wikimedia Commons.
City south of Hanoi, the gateway to the 'Ha Long Bay on land' karst valleys. Trang An, Tam Coc and the ancient capital of Hoa Lu all lie within easy reach. The city itself is a handy overnight base, with the karsts rising at its edge.