The giant central area was constructed in four concurrent building phases with annexed nave chancel and crypta. The original building remained in the ground floor zone and was visible, it was simply recovered and broken off in the upper sections, built up to an enormous height and domed with a decagon – a work of high architectonical achievement, which still today determines the appearance of St. Gereons. This construction, which is unique north of the Alps, has frequently been compared with the Hagia Sofia in Constantinople and the cathedral dome in Florence. St. Gereon in Cologne was very special for the architectural history of its time. The integration of the ancient walls into the new building structures was also a work of genius.
The individual phases in its history can still be well identified in the present. It is still possible today to perceive the cult of the soldier martyrs: some of the reused Roman sarcophagus’ are still filled with relics from this time. The view below up into the decagonal 21-metre-wide arched roof. It provides a fantastic space and light experience: the strongly coloured windows from Georg Meistermann created a light situation which seems highly similar to the Middle Ages - colourful and relatively dark. Great intensity also comes from the dome paintings in bright red with golden ornaments.