St. Gereon's Basilica in Cologne

You're standing at St. Gereon's, the oldest church in Cologne. It sits slightly above its surroundings, on a low hill. The two streets crossing here still follow the line of ancient Roman roads. Start with the most striking part: the big ten-sided building with its dome. It got its current shape between 1219 and 1227, but there's something far older hidden inside.