The postcard actually shows, I believe, the portals (or doors?). The Cologne Cathedral is the seat of the Archbishop of Cologne and is dedicated to Saint Peter and the Blessed Virgin Mary. In 1996, the cathedral was added to the UNESCO World Heritage List, being one of the best-known architectural monuments in Germany, and described by UNESCO as an "exceptional work of human creative genius".
Construction of the Gothic church began in 1248 and took, with interruptions, until 1880 to complete – a period of over six hundred years. It is 144.5 metres long, 86.5 m wide and its two towers are 157 m tall.
Cologne Cathedral Wikipedia Entry