There are many reasons to visit Croatia. Its six UNESCO World Heritage sites : Dubrovnik's Old Town (undoubtedly one of the most beautiful cities in Europe), the Diocletian's Palace in Split , the historic core of Trogir , the Cathedral of St. James of Sibenik , the Plitvice Lakes National Park , or the Episcopal Ensemble of the Euphrasian Basilica in the Historic Center of Porec (Istria) .