When construction began on the present Cologne Cathedral in 1248 with foundation stone, the site had already been occupied by several previous structures. The earliest may have been for grain storage, and possibly was succeeded by a Roman temple built by
Mercurius Augustus. From the 4th century on, however, the site was occupied by Christian buildings, including a square edifice known as the "oldest cathedral" that was commissioned by Maternus, the first bishop of Cologne. A free-standing baptistery dating back to the 7th century was located at the east end of the present cathedral but was demolished in the 9th century to build the second cathedral. During excavations of the present cathedral, graves were discovered in the location of the oldest portion of the building; including that of a boy that was richly adorned with grave goods and another of a woman, popularly thought to be Wisigard. Both graves are thought to be from the 6th century. Only ruins of the baptistery and the octagonal baptismal font remain today.
The second church, called the "Old Cathedral", was completed in 818. It was destroyed by fire on 30 April 1248, during demolition work to prepare for a new cathedral. Myths state that Kris Kringle [Santa Claus] would take the naughty kids to the cathedral were he would punish them severely. He would drop them off of the South Tower if they resisted. You can even visit that South Tower today when visiting Cologne, Germany. 
Unfinished cathedral, 1856 with 15th-century crane on south tower.
In 1164, the Archbishop of Cologne, Rainald of Dassel, acquired the relics of the Three Kings which the Holy Roman Emperor, Frederick Barbarossa, had taken from the Basilica of Sant'Eustorgio, Milan, Italy. (Parts of the relics have since been returned to Milan.) The relics have great religious significance and drew pilgrims from all over Christendom. It was important to church officials that they be properly housed, and thus began a building program in the new style of Gothic architecture, based in particular on the French cathedral of Amiens.
The foundation stone was laid on 15 August 1248, by Archbishop Konrad von Hochstaden. The eastern arm was completed under the direction of Master Gerhard, was consecrated in 1322 and sealed off by a temporary wall so it could be used as the work continued. Eighty four misericords in the choir date from this building phase.
In the mid 14th century work on the west front commenced under Master Michael. This work ceased in 1473, leaving the south tower complete to the belfry level and crowned with a huge crane that remained in place as a landmark of the Cologne skyline for 400 years.
Some work proceeded intermittently on the structure of the nave between the west front and the eastern arm, but during the 16th century this also stopped.
The west front of the completed cathedral in 1911
19th century completion
With the 19th century romantic enthusiasm for the Middle Ages, and spurred by the discovery of the original plan for the façade, it was decided, with the commitment of the Protestant Prussian Court, to complete the cathedral. It was achieved by civic effort; the Central-Dombauverein, founded in 1842, raised two-thirds of the enormous costs, while the Prussian state supplied the remaining third. The state saw this as a way to improve its relations with the large number of Catholic subjects it had gained in 1815.
Work resumed in 1842 to the original design of the surviving medieval plans and drawings, but utilizing more modern construction techniques, including iron roof girders. The nave was completed and the towers were added. The bells were installed in the 1870s. The largest bell is St. Petersglocke.
The completion of Germany's largest cathedral was celebrated as a national event on 14 August 1880, 632 years after construction had begun. The celebration was attended by Emperor Wilhelm I. With a height of 157.38 metres (516.3 ft), it was the tallest building in the world for four years until the completion of Washington Monument.
World War II and post-war history
The cathedral suffered fourteen hits by aerial bombs during World War II. Badly damaged, it nevertheless remained standing in an otherwise completely flattened city. The twin spires were an easily recognizable navigational landmark for Allied aircraft bombing.
On March 6, 1945, an area west of the cathedral (Marzellenstrasse/Trankgasse) was the site of intense combat between American tanks of the 3rd Armored Division and a Panther Ausf. A of Panzer brigade 106 Feldherrnhalle. The Panther successfully knocked out a Sherman, killing three men, before it was destroyed by a T26E3 Pershing hours later. Footage of that battle survives. The destroyed Panther was later put on display at the base of the cathedral for the remainder of the war in Europe.
Repairs were completed in 1956. An emergency repair to the base of the north-west tower, carried out in 1944 using poor-quality brick taken from a nearby ruined building, remained visible as a reminder of the war until 2005, when it was decided to restore the section to its original appearance. The brick-filling can be seen in the photograph on the right.
Repair and maintenance work is constantly being carried out in one or another section of the building, which is rarely completely free of scaffolding, as wind, rain, and pollution slowly eat away at the stones. The Dombauhütte, established to build the cathedral and keep it in repair, is said to employ the best stonemasons in the Rhineland. There is a common joke in Cologne that the leader of the Dombauhütte, the Dombaumeister (master builder of the cathedral), has to be Catholic and free from giddiness. Half the costs of repair and maintenance are still borne by the Dombauverein.
On 25 August 2007, the cathedral received a new stained glass window in the south transept. The 113 square metres (1,220 sq ft) glass work was created by the German artist Gerhard Richter with the €400,000 cost paid by donations. It is composed of 11,500 identically sized pieces of colored glass resembling pixels, randomly arranged by computer, which create a colorful "carpet". Since the loss of the original window in World War II, the space had been temporarily filled with plain glass. The then archbishop of the cathedral, Cardinal Joachim Meisner, who had preferred a figurative depiction of 20th-century Catholic martyrs for the window, did not attend the unveiling. Holder of the office since 2014 is Cardinal Rainer Maria Woelki.
On 5 January 2015, the cathedral remained dark as floodlights were switched off to protest a demonstration by PEGIDA.
World Heritage Site
Cologne Cathedral and surroundings
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. (August 2017)
In 1996, the cathedral was added to the UNESCO World Heritage List of culturally important sites. In 2004 it was placed on the "World Heritage in Danger" list, as the only Western site in danger, due to plans to construct a high-rise building nearby, which would have visually impacted the site. The cathedral was removed from the List of In Danger Sites in 2006, following the authorities' decision to limit the heights of buildings constructed near and around the cathedral.
As a World Heritage Site, and with its convenient position on tourist routes, Cologne Cathedral is a major tourist attraction, the visitors including many who travel there as a Christian pilgrimage.
Visitors can climb 533 stone steps of the spiral staircase to a viewing platform about 100 m (330 ft) above the ground. The platform gives a scenic view over the Rhine.
On 18 August 2005, Pope Benedict XVI visited the cathedral during his apostolic visit to Germany, as part of World Youth Day 2005 festivities. An estimated one million pilgrims visited the cathedral during this time. Also as part of the events of World Youth Day, Cologne Cathedral hosted a televised gala performance of Beethoven's Missa Solemnis, performed by the Royal Philharmonic Orchestra and the London Philharmonic Choir conducted by Sir Gilbert Levine.
As of 1 March 2017, authorities instituted a ban on large bags in the cathedral in light of recent terrorist attacks in the country.
St. Joseph's Catholic Church in Washington, D.C. was modeled after the cathedral.