Founded by the Romans over 2,000 years ago, Cologne is Germany's second oldest city. The city marked the northern boundary of the Roman Empire and was therefore of great strategic and military importance to the Romans. For many centuries now it has been a leading trading and transportation hub due to its location along the Rhine River. Cologne became one of the focal points of Western culture during the Middle Ages when, under the rule of Friedrich II, the relics of the Three Wise Men were brought to Cologne in 1164 after being captured from Milan. Cologne Cathedral, one of the most magnificent structures north of the Alps, was built to house these relics, although construction wasn't completed until 600 years later. Today the landmark is the emblem of Cologne and is also a World Heritage Site of UNESCO. The old town facing the Rhine together with the city boasting 12 Romanesque churches among its numerous other historical sites creates a unique ensemble and Cologne is also known as the center of the German Mardi Gras celebrations that peak during the carnival period.