Castlevania is a series that started 20+ years ago, and is still going strong today. Not many franchises can say the same thing.
Although I believe that if Castlevania went exclusively 3D with the advent of the N64, it would not be as popular as it is today.
Thank God for the GBA, DS, and PSP. They allowed for some of the most stellar CV games to date, and brought Castlevania back to 2D where it BELONGS!