Asturias: Home to the most legendary, terrifying, notorious climb in Spain, and perhaps all Europe: the Alto de l'Angliru.
This climb has an interesting history, as it's only been around since 1999. In fact, there was no road before then - the road was made specifically for the Vuelta a España. The race organizers wanted a road that would make their Grand Tour legendary, such as the Alpe d'Huez, the Tourmalet, or the Stelvio, so they created a road that would go on to become one of the hardest cycling climbs in the world.
This road has since been included in 7 Vueltas (most recently in 2020) but has not been without controversy. This climb is so steep, so brutal, that world tour cyclists have a hard time finishing it. Most famously, in 2002 Cofidis rider David Millar stopped a half-meter short of the finish line and refused to cross it in protest, exclaiming "We are not animals and this is inhuman!"
The best, hardest, most scenic and epic road bike climbs of Asturias can be viewed on this page via the map (zoom in to view climbs in particular areas), by Climb Cards (photo and stats) or by a detailed list of the climbs that can be arranged in metric or imperial units by difficulty, distance (longest climbs in the area), altitude gained, highest elevation (highest summit), average grade (sort the grades by varying distances), lowest start point, etc. Click on "Climb Page" to jump to the detail page for that particular bike climb. This page provides an excellent and comprehensive summary of the top, hardest and best bicycling climbs in Asturias.