forms of tourism have developed. The tourist gravitational area is considerably large, however the tourist market is small. There has been no large-scale tourism and no acute environmental pressures; in 2017, National Geographic Traveller's Magazine declared Slovenia as the country with the world's most sustainable tourism.The nation's capital, Ljubljana, has many important Baroque and Vienna Secession buildings, with several important works of the native born architect Jože Plečnik and also his pupil, architect Edo Ravnikar. At the northwestern corner of the country lie the Julian Alps with Lake Bled and the Soča Valley, as well as the nation's highest peak, Mount Triglav in the middle of Triglav National Park. Other mountain ranges include Kamnik–Savinja Alps, the Karawanks, and Pohorje, popular with skiers and hikers. The Karst Plateau in the Slovene Littoral gave its name to karst, a landscape shaped by water dissolving the carbonate bedrock, forming caves. The best-known caves are Postojna Cave and the UNESCO-listed Škocjan Caves. The region of Slovenian Istria meets the Adriatic Sea, where the most important historical monument is the Venetian Gothic Mediterranean town of Piran while the settlement of Portorož attracts crowds in summer. The hills around Slovenia's second-largest town, Maribor, are renowned for their wine-making. The northeastern part of the country is rich with spas, with Rogaška Slatina, Radenci, Čatež ob Savi, Dobrna, and Moravske Toplice growing in importance in the last two decades. Other popular tourist destinations include the historic cities of Ptuj and Škofja Loka, and several castles, such as Predjama Castle. Important parts of tourism in Slovenia include congress and gambling tourism. Slovenia is the country with the highest percentage of casinos per 1,000 inhabitants in the European Union. Perla in Nova Gorica is the largest casino in the region. Most of foreign tourists to Slovenia come from the key European markets: Italy, Austria, Germany, Croatia, Benelux, Serbia, Russia and Ukraine, followed by UK and Ireland. European tourists create more than 90% of Slovenia's tourist income. In 2016, Slovenia was declared the world's first green country by the Netherlands-based organization Green Destinations. On being declared the most sustainable country in 2016, Slovenia had a big part to play at the ITB Berlin to promote sustainable tourism. Transport Since Antiquity, geography has dictated transport routes in Slovenia. Significant mountain ranges, major rivers and proximity to the Danube played roles in the development of the area's transportation corridors. One recent particular advantage