Overlooking the vineyards of Goriška Brda stands a modest but elegant castle called Gredič, the story of which is key to the transformation of these villages from an impoverished backwater to one of the world’s most prized wine centers. Goriška Brda is a cluster of 45 villages spread over 72 square kilometers (28 square miles). The region has a special terroir known as opoka in Slovenian or flysch in English, that carries over across the modern border with Italy into the Italian region of Collio. This was a single region from…