Here are two photos of the very first Lego castle, made in the early 1970s as a promotion for the 'Weetabix' breakfast cereal in the UK. I remember collecting coupons for it and the postman delivering the parcel, but can't remember what the box was like. There is one very, very small piece missing from the set :(
If you've got any other information about this set(such as exactly which year it was released) please let me know, or visit Ben at Classic-Castle.com, There is also a low-res colour scan of the manual at Isodomos.com.
(and yes, this IS important!)
