I disagree with the placement and commentary around
Castles. Castles are usually a more efficient way to green than Provinces. If the game ends on piles, neither pile is very relevant, but Provinces are a little better. But if the game is going to last a while, and there isn't a Province-emptying megaturn, you will probably lose if you ignore Castles. The full Castle stack (in 2P) is worth 45+ VP and is significantly easier to obtain than 8 Provinces (worth 48 VP). More realistically, you trash a couple Castles with Small Castles and grab a convenient Province to have a ~44 VP to 42 VP lead over a player who gets 7 Provinces.
You can start gaining Castles ever-so-slightly before you would gain Province, because the early Castles provide some payload. Of the early Castles,
Humble Castle is the most important. This $3 card can easily be worth as much as a Province if Castles aren't contested, and is still great value if you only manage to hold onto 4 Castles by game's end.
Crumbling Castle is the weakest castle; but it's okay if you have overdraw.
Small Castle is a key card if Castles are contested, and it's generally an efficient way to gain the later Castles. Crumbling Castle is the best target, but Haunted Castle and Small Castle itself are also fine to trash.
Haunted Castle is usually great; it attacks your opponent and provides some payload to afford the later Castles. In decks with good draw and available +actions,
Opulent Castle can be fantastic; it gives you the coins you need as you fill your deck with green.
Sprawling Castle,
Grand Castle, and
King's Castle are straight points, and competitive or superior to Provinces. These are the cards you want to use your Small Castle to gain.
I had Castles up on rank 17 of my list, and I think it's being underrated. Similar to
Fairgrounds, Castles adds a significant chunk of VP to the game, which rewards building longer. Castles are almost always good on a board if Fairgrounds would be good. While Castles can't be as insane as Fairgrounds in the presence of lots of cards outside of the kingdom (see
Black Market), Castles are better any time it's inconvenient to have 15+ unique cards on one's deck - which is most of the time. I also give Castles the nod over Nobles. If
Nobles is the only source of draw in the kingdom, it can be a key card, but otherwise it's not as impactful as Castles; it represents only 16 extra VP. I even have
Dominate below Castles, because Dominate is an impractical target on a substantial fraction of kingdoms and it works on the shared Province pile.