Cinderella's castle does not dominate Fantasyland as you might expect. Instead, it is the centerpiece of the entire park, the hub around which everything else revolves. Main Street, then, does not open up onto the county courthouse, but onto a medieval castle.I noticed the same thing last year when my family was there, but I attributed it to Disney marketing its relatively current Pirates of the Carribean movies to boys. Unless and until they start cranking out movies about knights, it'll be pirate toys that attract the boys (aaargghhhh!!!).
It would be wrong, however, to over-read this as a medieval image. Instead, I think the dominant idea is one of the fairy tale, which is then associated with medieval architecture. The park is innundated with fairies and princesses, but you're hard pressed to find a knight, or happy peasants working the fields, or a monastery, or any of the other popular images associated with the Middle Ages. Fantasyland does have Excalibur in a stone that you can pose with, but there is little else Arthurian.
Consider too the cosplay. Little girls are dressed like fairies and princesses, but little boys dress as pirates -- not princes or knights or kings. Swords are either clearly pirate cutlasses or lightsabers (for nighttime play) -- but consider all the various Prince Charmings, and Robin Hoods, and other fairytale male characters that boys could be dressed as.
Disney almost exclusively (and successfully) uses the princess motif to portray heroic girls/women in their movies and the easiest primary source from which to cull ideas to keep the revenue stream flowing is in fairy tales. Some of the best known fairy tales revolve around princesses and the themes of those tales still appeal to a broad audience (and I've got two of them!).
But I wonder what Disney's parks would look like had they developed Shrek? I know I've seen a lot of boys with the Shrek masks and, though not necessarily a role model for kids, I bet Disney's marketing department could have had some success with Puss-in-boots. So, maybe its only Disney-fied fairy tales that are for girls.