The market town of Castle Douglas, beside Carlingwark Loch in the southern Scottish region of Dumfries and Galloway, is relatively new, though the area has been inhabited from prehistoric times and the Romans had a military base close by. In the fourteenth century, Archibald the Grim, the 3rd Earl of Douglas, built Threave Castle nearby. The town came into being thanks to fertiliser found in the loch and wealthy merchant William Douglas, who laid...