Split piles are those containing more than one differently named card, in a particular order. This is in contrast to Knights, which are shuffled at the start of the game. Typically, the cards in a split pile have some particular synergy with each other.
List of Split pile cards
/, /, /
Castles, /, /
So how do these work exactly? The top cards are paired with the bottom ones. Each pile has 5 copies of the first (cheaper) card, then 5 copies of the second (more expensive) card. You can only buy/gain the card on top; if you want the other card, you have to dig it out. There are special randomizers for these piles, that list both cards and have art showing both things. You will see those when the set comes out; the six actual cards will have to do for today. And uh some things in the game care about stats for a pile, and well they go by the randomizer, which usually matches the top card. can have Gladiator/Fortune as its bane (and then both cards will do the trick), lets you put your + token
on the Catapult/Rocks pile (and then both cards give + ), and so on.