Not sure it this counts as a castle set, but I bought 25 Spartans from Collectible Minifigures Series 2.
I also bought 13 Court Jesters
and 8 Axe Carts
Moving on to larger sets, I bought at least 4 copies of Medieval Market Village
. Possibly more if there are any hiding in my closet. I also bought 5 or more copies of Legend Black Falcon's Fortress
. (actual number unknown.)
Before my dark ages (1992-2002), I bought multiples of only two Castle sets because I didn't know it was okay to buy multiples. They were Battle Dragon
and Black Monarch's Ghost
Generally speaking, I bought sets in multiples for parts/figs or because I got them at a great price.