Grand Castle just mentions Victory cards "in play", not that *you* have in play. Does that mean that if my Platinum is Swindled into a Grand Castle on my opponent's turn, and he has a Great Hall in play, I should get +1 from his Great Hall, in addition to the from any Victory cards in my hand?
Yes.
So if I have my Estate token on a Fishing Village (or other Duration), and you buy Grand Castle, you get +1 VP for every Estate I have in play? I would have thought "in your hand and/or in play" was short for "that you have in your hand and/or have in play". That's just the common sense interpretation (according to my sense of English), but I guess you're going for the literal parsing here.
I have chosen to go with the literal interpretation.
I see. That made me wonder about "in play" though.
If "in play" considers all play areas (rather than just the player who is addressed by the ability), what makes Lighthouse just work for the player who has it in play, rather than all the players? I mean, a card with "when you buy/gain this" works for any player. Why doesn't "when another player plays an Attack card" work for any player in the same way (as long as Lighthouse is "in play")?
Cause it explicitly says: 'doesn't affect you'?
This sounds right. "Another player" is defined as "a player other than the player who played this card." And "you" is defined as "the player who played this card." So Lighthouse reads as:
"While this is in play, when a player other than the player who played this card plays an Attack card, it doesn't affect the player who played this card."
I think the question is why some abilities (Lighthouse, Goons, Talisman, etc.) operate in a context where "you" means a specific player, while other abilities (Border Village, Farmland, Duchess, etc.) operate in a context where "you" is shorthand for "any player." And where they get that context from.
Imagine that, someday, there's a way to get any card into play without having played it. "Choose a card in the trash and put it in play without playing it" (don't make that card, Donald). So, the "while this is in play" abilities should still work in general, right? I mean, nobody would have any problems with Highway there. But then what would Lighthouse do? Who would "you" be at that point? Do you say "Oh, I guess 'you' is now also the player who puts a card into play"? Would Lighthouse's ability even mean anything in that case? (Those are rhetorical questions, by the way; not asking for an actual ruling.)
It's almost like stuff like Lighthouse's second ability should actually be above the horizontal line. So they would set up that state while they were still in a context where "you" meant the specific player who played the card. And if they somehow got into play without being played, no big deal; the state that would refer to "you" never even got set up. (That interpretation would make the Hot Potato thing Jeebus posted while I was typing this not work, though. The Hot Potato would probably have to say "while this is yours" or something.)
EDIT:
Actually, Haunted Woods and Swamp Hag already do things that way. Above the horizontal line (well, ok, there is no horizontal line), they set up a state that refers to "any other player." So they do that in a context where that concept is meaningful (because "you" refers to a specific player).