Kölner Dom is the German name for the Cologne Cathedral... but calling it Cologne makes me feel like an ignorant American tourist, so I much prefer the name Köln. Besides, the Dom, pronounced 'Dome' makes it sound to me like a giant sports stadium or death match Mad Max arena, which is much more exciting. Back on topic, the Kölner Dom is the single most visited landmark in Germany, receiving 20,000 visitors a day according to Wikipedia! 20,000 seems like a lot, but I've been there three times now, and each time it has been pretty busy, for what it's worth.
I keep going back because, well, firstly it's not very far from where I work in Essen, and secondly because I still haven't gotten a really good photo of it! Built in the Gothic style, it is very dark inside and hard to photograph without a tripod, which I never carry when travelling anyway. I'd also like to try again with a wider lens, such as the nice Fuji 14mm which I still can't afford yet! This trip I arrived just in time to make one of the twice-daily English language tours, which I hadn't done previously, and it was good value for $6 because we got to go inside the choir, which is not accessible without a tour, and see the famous reliquary up close, and get some nice photos from the centre of the building. The 'Shrine of the Three Kings' reliquary supposedly contains the remains of the 'three wise men', commissioned by the Archbishop of Cologne around 1190ad.
The Dom is very impressive both inside and outside, covering a huge land area and is also very tall... you can also climb the 509 narrow, winding stone steps up the tower, but I've done that before and once was enough! It is the largest Gothic church in Northern Europe, and with the two huge towers has the largest facade of any church in the world. So for anyone interested in Cathedrals and Gothic architecture like me, this is a pretty good example and well worth visiting (several times).
The Dom construction was first started in 1248ad, being built continuously until 1473, and then stopped until the 19th century, and not finished until 1880, 552 years after it was started! At that time it was the tallest structure in the world, for only four years before the Washington Monument was erected. It was struck by 14 bombs in WWII but stood strong in the otherwise almost completely flatted city, although it is still under continuous repair and reconstruction of the stained glass windows in particular since the war. It is now a listed World Heritage site.