I'd like to give my own take on the Castles, which includes a lot of agreement, but some disagreement. I'll be using
play stats markus has collected to support my points.
I mostly agree with your high level thoughts on
Castles. The way I would describe in which games to go for Castles is: any game when you would otherwise buy Provinces over multiple turns. This is a slight majority of kingdoms. Castles are a slightly better source of points than Provinces in these games, but you'll usually want a mixed greening strategy. Castles can usually be ignored on megaturn boards (think
Bridge), boards with exceptionally strong Alt VP (
Goons with support, Colonies), and boards that are going to end on piles quickly. Castles are stronger Alt VP than
Fairgrounds, and ought to be gained more often. The
winner gains at least one Castle in 58% of recorded games, and the pile empties in ~26% of games.
Compare with Fairgrounds, where the winner gains at least one Fairgrounds in 29% of games, and the pile empties in only ~7% of games.
When looking at the individual Castles, I'd like to start from the end. You say: "[Opulent Castle is] also the only Castle that is definitively a good card for you if you are otherwise ignoring the Castle pile and plan to win on Provinces." I think this applies best to the last three Castles in the pile, which are all Province substitutes:
Sprawling Castle,
Grand Castle, and
King's Castle. Sprawling Castle is worth at least as much as a Province + Estate, and can be worth 1-3 VP more depending on whether you have Crumbling Castle or get King's Castle. Grand Castle is almost always at least as good as Province in straight points. King's Castle is usually a 15 VP swing from one player to the other (sometimes 17 VP, rarely 13 VP or less). Province is a 12 VP swing, so King's Castle is also often better than Province. Gaining each of these three Castles is positively associated with winning the game. The player who gains
Sprawling Castle wins 56% of the time;
Grand Castle: 64%,
King's Castle: 68%!
I'm not as high on
Opulent Castle as you. In my experience, it tends to be revealed late enough in the game that I can't get great value out of it's on-play ability. It anti-synergizes with sifting. For all that, it is a solid card, and you'll know the games in which it's going to be unusually good. In general I wouldn't worry about leaving it for the opponent.
The player gaining Opulent Castle wins 55% of the time.
Haunted Castle is better than you give it credit for. The on-gain attack is great at causing a partial dud. Even without the excellent attack, Haunted Castle is a Harem/Conquest variant, and better than both in the presence of sifting, remodelers, or any Castles synergy.
The player who gains Haunted Castle wins 57% of the time.
Small Castle is usually strong; it gives a lot of endgame control, as long as you have the +actions to play it. Crumbling Castle and Haunted Castle are the best targets. I would almost never trash Humble Castle over the Small Castle itself (Humble Castle is always 2+ VP and $1 if you have this choice). Small Castle has additional synergy with cards that recover it from the trash, like
Rogue and
Lurker.
The winner gains Small Castle 54% of the time.
Crumbling Castle is indeed the weak link in the Castle line. It is the only Castle gained more often by the loser than the winner (
48% by the winner). Adding a money density of $1/card from a flat 2 VP is a bad deal at almost every point in the game. Still, it has its good cases. If you can trash the Crumbling Castle for benefit, it can end up better than a Conquest/Harem (and cheaper!). On some Donate boards, a pre-Donate Crumbling Castle buy can be worth playing for/around.
Humble Castle is where the pile starts. This card is powerful for its cost, but often gained too early.
The player who gains it wins 53% of the time, but the loser is more likely to gain it before turn 10. Hovel and Keep can tempt players to open Humble Castle, but even in these cases you should strongly consider your other options. Opening Humble Castle for Keep is especially dubious.
People tend to gain Castles too early. Yes, the early Castles are better for your deck than straight green cards, but not better than other cards you could be buying. In fact, because Castles will increase the amount of VP before the game ends, you might not want to get Castles until you would otherwise buy Provinces if Castles weren't there. I agree with ehunt that getting 3-4 Castles in one turn is an excellent start to greening. With Humble, Crumbling, and Small Castles, you force your opponent to contest by buying some Castles, which are less synergistic for their deck. If your opponent goes off pre-cocked and picks up just Humble Castle and Crumbling Castle: grab Small Castle and Haunted Castle and be happy. In all of these cases, prepare your deck for the extra stop cards. Crumbling Castle, Haunted Castle, and Sprawling Castle in particular add two stop cards each.