Two miles west of Castle Douglas, this impressive tower sits on a small river island. Built in the late 14th century, it became a principal stronghold of the Black Douglases, including the excellently named Archibald the Grim. It's now basically a shell, having been badly damaged by the Covenanters in the 1640s, but it's a romantic ruin nonetheless. It's a 15-minute walk from the car park to the ferry landing, where you ring a bell for the custodian to take you across.
Also from the car park, where there's a small nature exhibition, a 1.5-mile circular nature path gives you the chance to spot deer and ospreys, as well as waterbirds from hides. At dusk it's good for batwatching.