The red brick and white pointing gives an initial impression of modernity.  This looks very like the kind of construction we see in American urban row-houses.   Don't be fooled.  This church was built in 1170 AD by Bishop Absalom.  It is the first Gothic-style church done in brick, all the others having been done with granite or similar stone.

Look closely at the stone flooring.  They are not simply slate plates — they're inscribed.  They're tombstones.  The custom in those days was to bury an important person within the church — often beneath one of the aisles — and to make the tombstone a floor tile.

When this church was built, Roskilde was Denmark's capital, the seat of government.  From about the 15th century on, all of Denmark's rulers have been buried here.  Some of the tombs are very impressive. 

This is the tomb of Queen Margrete I.

Most Gothic cathedrals are very high and very poorly-lit.  Roskilde Domkirke is neither.  Decoration on the ceiling can therefore be enjoyed by the worshippers, and becomes a worthwhile effort.  But notice, in the leftmost panel, the five-petaled rose (an icon of Lutheranism) and the absence of anything 'strictly Catholic' that tells us this ceiling has been redesigned and repainted after the Reformation.