The jewel of Cologne (Koln), Germany, is the Gothic cathedral that is now a UNESCO World Heritage Site. Construction started in 1248 and didn't complete until 1880. The Dom is the 3rd largest Gothic cathedral in the world. It was heavily damaged during World War II, but brought back to life over the years. It's hard to explain the pure massiveness of this place. During a recent visit to Cologne, I photographed inside using my iPhone. The cell phone does a great job capturing inside in low light that just can't be done with a DSLR without a tripod.
DSLR shot earlier in May
One of my DSLR shots from 2018

Mosaics
Treasury
St. Maria Himmelfahrt Church
We refer to this one as the pink church. With it's Baroque style architecture, it is probably the next best church in Cologne.
Church of St. Agnes in the old part of Cologne is a neo-Gothic Catholic church dating back to 1902. I loved the steeple, but the sanctuary was a little less ornate but still beautiful.