Well, likely many people do realize that the Disney castle is not representative of real castles - although quite likely less than you suspect (which has less to do with the average intelligence of Americans, but the average intelligence of people in general). And even if they do realize it, how many of them know what a real castle looks like? In the RPG community we take such knowledge for granted, but how common is such knowledge of history, and especially European history, in the general American population?
Even if they don't think of the Disney castle, the best image that will pop to their minds are likely Minas Tirith and Minas Morgul from the Lord of the Rings movie. And knowing anything better is not a fault of their character, but simply a matter of having different interests than the average gamer...