The old King's Road across Filefjell. For as long as people have lived in Norway, this has been the main route between east and west. The geography explains why: from the coast you could sail up the Sognefjord, find a gentle valley at Lærdal leading to Norway's lowest mountain pass, then descend through another easy valley in Valdres. No other crossing made as much sense.
The road is named after King Sverre, who marched his army this way in 1177. It became an official postal route in 1647, but remained a rough bridle path for packhorses. That changed in the 1790s when engineers built a proper four-metre-wide road using the "French principle" - straight lines wherever possible, solid foundations, good drainage. It was the first road where you could drive horse and cart between Oslo and Bergen.
There's a dark footnote. Locals in Valdres and Lærdal had been exempt from military service for centuries in exchange for maintaining the route. When the new road was finished in 1793, the king ended this arrangement. The people of Lærdal revolted. The army and navy were sent in. The rebellion's leader, Anders Lysne, was executed by decapitation.
Today the historic road has been restored as a 100 km hiking trail, split into 12 sections. It takes three to six days to walk the whole thing. The route won the Europa Nostra Award in 2017, plus the Grand Prix - one of Europe's most prestigious cultural heritage prizes.