Considered a world heritage site under UNESCO criteria, Kölner Dom, as it is known to the Germans, is one of the biggest churches in the world and the most visited landmark in Germany.
This masterpiece of Gothic architecture began construction in 1248 and was finally completed in 1880 after a being left unfinished for a few centuries. The end result was worth waiting for, for the cathedral has twin spires reaching 157 meters (515 feet) in height, making it the largest facade of any church on Earth. Inside, it houses eleven church bells (four of which are medieval), a golden sarcophagus said to hold the remains of the three wise men, and a black marble altar nearly 800 years old. That's just the start; there is also quite a collection of shrines, stained glass, and statues representing various religious figures and scenes.
Admission to the cathedral towers is about $4 USD and is fully worth it for the view of the city, while guided tours are conducted numerous times throughout the day. Suffice it to say, if there's one German Gothic cathedral on your must-see list, it just may have to the Cathedral of Cologne.