Barcelona, Spain

Barcelona is the world's most architecturally coherent city—its Eixample district, a perfect 1860 grid by Ildefons Cerdà, hosts 9 UNESCO World Heritage buildings by Antoni Gaudí alone, the highest concentration of a single architect's works globally. Gaudí's Casa Batlló and La Sagrada Família exemplify organic modernism, drawing 4.5 million visitors annually.