Triumph seems okay, but as mentioned should be a looter instead of having that setup clause and say "less than it" instead of "less than this".
Haunted Castle would seem okay at $6 without the -VP. Hireling is not "too strong". It's a Lab you'll play every turn - which a good engine can have for $5 (by getting a normal Lab). Sure, Hireling also helps for other strazegies than engines. That's why it costs more. It's also useless on the turn you play it. Either way, you need to decide whether you want to say "next turn" or "each of your turns". "Each of your next turns" ddoesn't make much sense. I don't think giving the card the curse type is that good, either way. There are other ways of making a card give negative VP, such as giving other players VP tokens on buy/play. Again, not that i think it's necessary. The card seems weak allready.
Why do you make a card that's worth negative points and makes it really easy to trash itself? That's kind of a moot point. Also i advise to take a look at the "fan card creation guide". It points out that negative VP should probably scale with the use of a card, such as with the suggested VP tokens for opponents (or you gaining Curses/Curse tokens) on play. Be ause, if a card with -VP printed on it is just strong enough to be played in a game wizh good trashing, it will be horrible in games without. If it is good enough for games without, it will be a must-buy in games with trashing. Hardly balancable. As said, with Imp the VP are moot either way. Also: The wording should say to trash a card "from it" to make clear the trashed card comes from the discard pile.
Provincial Revolt is a Saboteur variant that only targets VP cards. Being a mirror image of the dreaded Rebuild, i can't claim i like it much. The worst part is that it actively discourages getting VP cards, an in fact discourages to move the game along at all. And that not only with its attack, but also the nonterminal VP token. Imagine me buying only these. How would you be possibly able to win if i always return the Provinces you bought, getting points at the same time? Just playing 1-2 of these a turn would be a certain win. So, to beat me, you have to do the same. Which in turn means, the game never ends.
Saturnalia is much worse than Gold. Sure, there's a card, but a single card doesn't help much on a terminal. All it does on Saturnalia is draw stuff dead that you will have to discard. I'd argue it actively harms you most of the time. So, this takes up two more actions than Gold, can be drawn dead (by Saturnalia, for example) and gives me what in turn? A buy? You could also shorten the wording to "-1 Action (to a minimum of 0)".
I think Siege Machine really shouldn't either hit double or not at all. Rather do it like Giant and give out Ruins if it misses. I know that's less thematical, but theme is something that can make a good card better, not something that can make a card good. Here the theme seems to work against the card.
Sorry for all that critizism. Also, ignore the thing about prize convexity. That was an inside joke based on something that happened on these forums recently.
And last, but not least, welcome to the forums!