My thought was that Road Network happens to Alice, since it triggered when she gained a Victory card.
You have ruled that when Alice buys Noble Brigand with Embargo token, she gets to choose which to resolve first, since Noble Brigand's "when you buy this" means that it happens to her. I assume it's the same with gaining Ill-Gotten Gains: "When you gain this" means it happens to her, so she can choose to use Watchtower before or after Bob gains a Curse. It happens to Alice even though the only thing that actually happens when she resolves it is that Bob gains a Curse.
So it's not the actual ability that decides who it happens to.
Road Network says "when another player gains a Victory card". "Another player" here is Alice. The only difference between this and Ill-Gotten Gains is that it says "another player" instead of "you". The actual ability is the same: Bob does something, Alice does nothing.
So is it different because it says "another player" instead of "you"?
Swamp Hag says "when another player buys a card". If Bob buys a card, he is the "another player". He resolves it since it's not optional.
Road Network: Alice gains a Victory Card, she is the "another player". So... she resolves it since it's not optional. Right?
What am I missing here?
I am going to try to figure it out, in case that's different from whatever previous rulings.
Let's look at the rulebook.
When two things happen to different players at the same time, go in turn order, starting with the player whose turn it is. For example, when a player plays Witch, the other players gain Curses in turn order, which may matter if the Curses run out.
When two things happen to one player at the same time, that player picks the order to do them, even if some are mandatory and some are not. This can come up with expansions.
This talks about things happening. It makes it sound like all we care about is the effect. And that's backed up by the Witch example.
But we may have to order things that themselves need ordering. Let's say I have "When you buy a Silver, each other player gains a Curse" and "When you buy a Silver, each other player gains a Copper"; do they each gain a Curse and then a Copper, or do I pick which to do first and then they go around?
I think it has to be that somehow we first order those two things, Curses or Coppers first, and then resolve them and that hands out cards in turn order as normal. That's my ruling on that corner of this. When you've got "when x, do y," you first order that relative to other "when x," and then do all of one y, then all of the next y.
When we're ordering those two things, well we are ordering them by the trigger; the effect doesn't provide a way to order them. In this case it seems simple, it's "when you" so "you" pick an order. An ability like "When x, gain a Gold and the player to your left gains a Curse" also needs to be ordered by the trigger (and then just sequentially once resolving it, the Gold is first).
Let's say instead it's "When another player buys a Silver" for the Curse one. So now we have to order, "when another player buys a Silver" vs. "When you buy a Silver." That's the Haunted Castle / Road Network example of course.
Hmmm can we remove the players. Let's have a below-the-line ability, like "setup" but not using that term as we want it to be someone's turn and not have to rule that it's the first player's turn during setup because that's just besides the point here. "At the start of the first turn in a game using this, each player gains a Curse." And then another one of those for Coppers. Well the timing rule simply doesn't cover this case. If it came up with real cards though my ruling would be that the player whose turn it is orders them. I guess it actually comes up with setup rules, e.g. Young Witch vs. Black Market. You could resolve Black Market's setup first and have no cards left over for Young Witch. I feel like in the murky past I've ruled that you do Young Witch first. Which uh. I mean if it were a more common thing we would want a general rule and it would either have to be "the first player picks even though what's up with that" or "can't you all just agree somehow." It wouldn't be "consult this list of how to order setup abilities."
Another thing to poke at is the source of the ability. We can make the source be non-player-specific, like Duchess's ability. I mean that happens to someone, but it's no-one's card causing that. Or hey Landmarks. Since we won't necessarily have "who had this ability" to go by, it would be great not to ever go by it. Let's not. That's another ruling, we're going places.
The implication of the timing rules is that everything happens to players. That's not true, there's e.g. "put a token on the Temple pile." We have to either say "we're counting that as happening to the player told to do it" or "now here's the timing rule for things that don't happen to players." Well that timing rule is "the player whose turn it is orders them" and I even already have "between turns go by the most recent player to take a turn." Anyway I think what makes sense here is to go by the player told to make something happen. I think that's more suggested by the rules than "some rule not there at all" is. The rule not there at all applies only in cases where absolutely nothing else covers it, e.g. ordering setup rules. This isn't me changing my mind about Duchess above; we don't care that Duchess's rule is on a supply card, what matters is that it addresses someone gaining a Duchy.
So uh. Haunted Castle is happening to Alice. Road Network is happening to Bob. We go in turn order. Alice's gain triggered Road Network, but Bob is the one being told to do something. Haunted Castle is telling Alice to do something (which includes, having each other player be attacked).
I am open as always to arguments about how wrong I am, what I missed, and why it should be otherwise. I have put some time into it today and that's how it looks to me.
Embargo / Noble Brigand. That ruling sure seems compatible. I buy Noble Brigand, I order it vs. Embargo. I am being told to do two things.
Swamp Hag / Noble Brigand. I buy Noble Brigand, I order it vs. Swamp Hag.