Basilica of St. Francis Assisi built in 1237 is Kraków's the most colourful church. It is well known for the eight stained-glass windows and the gorgeous Art Nouveau interiors created by young Cracovian Stanislaw Wyspiansky's. The colors and saturation make the interior of basilica similar to mosque.
There is a one of his famous works at the Basilica entrance named "God the Father in the Act of Creation" or just "The Creation" which shows God emerging from the cosmos in the act of creation.
Dating back to the 13th century, St. Francis' Basilica was the first brick building in the city and is well worth popping in.
Wikipedia
The Church of St. Francis of Assisi with Monastery of the Franciscan Order (Polish: Kosciol sw. Franciszka z Asyzu) located in the Old Town district of Krakow, Poland, is a Roman Catholic religious complex on the west side of All Saints Square at Franciszkanska 2, across the street from the Bishop's Palace – residence of Pope John Paul II during his stays in the city. The Church dates back to the 13th century. Saint Maximilian Kolbe was a monk there in 1919, and led his first service at this church on Poland's return to sovereignty.