The first Castle sets I ever owned were the Forestmen's Hideout and the 1988 Castle Mini Figures.... Unfortunately, they got given away, along with most of the rest of my LEGOs when I went into my first Dark Ages from 1995-1997... The first Castle Sets I got when I came out of my Dark Ages were the Royal Knights 6078 Royal Drawbridge, 6036 Skeleton Surprise, and the Royal Knights Catapult that was part of the 2-pack with the Spyrius spaceship (I combined parts from the two and a Town helicopter to make a fantasy-themed Gyrocopter, sort of like in Warhammer or Warcraft II)...
The OLDEST Castle set I have is the 6057 Sea Serpent, which I won in 1997 in an online auction.

I also managed to find a box of the 1994 "Medieval Knights" minifigure pack at a local retail store, which is the source of my lone Wolfpack Renegade. I also managed to find a couple Dragon Knights Crossbow Carts as well... I also somehow managed to acquire at least two Crusaders.... Possibly thanks to the 1998 re-release of the Treasure Cart.