Douro is Portugal's most internationally renowned wine-growing region, primarily due to the region's world-famous port wines. Port wines are grown on around three quarters of the 40,000 hectares of cultivated land. But the dry red and white wines of the wine-growing region have also gained in popularity over the past few decades. The steep slopes of the Douro valley, which is traversed by the river of the same name, make wine cultivation difficult and only a few wineries can be visited in the region itself. Interestingly, the region is one of the oldest legally defined wine-growing regions, with boundaries already being defined in the mid-18th century.