Todays stage finishes at Lagos Covadonga, I went up it last year, in a car I should add, but it is one hell of a climb. IMO it is probably harder than many alpine climbs, it is not a steady gradient, it is very steep in places and there are even a couple of downhill bits in the middle of the climb. I don't know if they go all the way to the very top because that is a bit restricted for cars etc., but it is a very hard climb.