Prepare
Move your Prepare token to a kingdom supply pile. (When any player (including you) gains a card from that pile, +1 Coffers.)
Event
$4
I think this concept has massive potential. The interaction of giving yourself a bonus when other players gain a particular card forces some interesting decisions for other players. However, the concept is hampered a lot by the fact that it triggers even when you gain a card from that pile. The decision for your opponents to buy the card or not is less interesting because you get the benefit even if you yourself buy it. I think this card would be better if you it was “when another player” instead of “including you.” It also prevents the balance issue of putting it on the only village in a game for a guaranteed +10 coffers.
I think the phrasing for target should be “when you would gain a card from this pile.” That has a precedent in Dominion and introducing an “immediately” still doesn't make the card play the way you want to. All effects happen immediately and you choose the order-- that's a core part of triggered effects in Dominion. “would gain” would have the same effect except for with trader allowing you to gain a silver instead of trashing the card, if you want. It would be virtually identical to what you have written, except you don't need to introduce the complexity of a new concept of “immediately.”
Ambush sets up some interesting possibilities. You can pile out the curses without gaining a single one. You can target cultists to get +3 cards when you gain one. You can buy any attack for 2 if you Target squire. I feel like most of the time those fun combos won't exist – it will play more like embargo. What cards does my opponent want that I don't? That's a really awesome decision. Notice that embargo is one-use only. I worry how easy it is to play these. An engine player can lock big money out of buying golds. And you get to play each ambush twice. There would be two ways you could improve the card and force more interesting strategies:
1) you could make Ambush say “move Targeted onto any non-Victory Supply pile that is not currently Target.” Now you are stuck with a dead card in your deck if you don't want to move it, or you have to build your deck to consistently play two of these. It makes the locking out a player a little more challenging.
2) you could make Ambush trash itself on play. (Still it's not too weak, you get two plays out of it since you gained it and trashed it).
This card really forces you to think of your opponents. In some games with lots of villages, you'll just be giving out villagers to your opponents. However, your opponents can anticipate that and waste one less of their buys on villages. So then perhaps you don't give them villages. This causes really thoughtful interactions. The only concern is that it does appear be weak, but it's fine because you probably don't want too many of these in your deck, anyway. It's a helpful way to smooth out your deck and the fact that it is non-terminal makes it like a really easy thing to slip in your deck. So, I anticipate this being something I'm happy picking up on a 2/5, but I'm rarely going out of my way for one. Not all cards get to be the strongest, this one certainly is interesting.
This is in the top 5
Exhibition
$2 - Night/Duration
Set aside an non-Duration Action you have in play. Until your next turn, when another player gains or plays a copy of it, they get +$1. At the start of your next turn, play it.
-
This is gained to your hand (instead of your discard pile).
I first want to analyze this card without the “penalty” of giving $ to your opponents. It is sort of like you played a caravan and a scheme the turn you play an Exhibition. Another way to think about it is it's a much stronger haven for action cards. (you get to play the action card both turns!). So, with that, the card is worth 3-5 without the penalty, so 2 with the penalty feels right. But this penalty encourages mono-strategies across the game which is a form of not-solitaire, (copy your opponents) but it's a decidedly, less fun version of not-solitaire. Of course this card is a lot better if your strategies differ. Or if you have a prize or a black market. In addition the +1$ is a pretty strong penalty, and you're going to want to put it on a card you want to play a lot to make the most of it – but that also makes the penalty stronger. This card works best if you can put it on cards you only want one of. For example, using Exhibition on a terminal trasher is probably best (chapel being the best – the +1$ is unlikely to benefit your opponent the turn they play a chapel). That works well in the beginning of the game, but this could turn into a dead card in many games (the +$ is just not worth it).
Really interesting decisions, I don't know how I would change this.
I feel like you unnecessarily added “gains or trashes.” In games with trashers, this is a super cheap terminal gold, with the +buy to immediately buy more of them. The +buy combos well with gaining, I think this card is best and most interesting and more focused if the Ale token only fires on gains. That way it isn't so easy to get this easy of a gold in the beginning of the game. The decisions later – “where do I put my token, and the opponent's decision, can I delay buying this card to keep their bartender in the tavern?” That is a super interesting decision and really shows how Dominion is not solitaire. Excellent idea.
This is in the top 5
There's a reason why mountain pass is only once per game. Bidding takes some time. This card creates bidding up to *twenty*
times in a game. That's a lot of bidding slowing down the game.. Sure, the trashing mechanism can slow this down, but I think if you have to design a card to trash itself so it isn't played so often, you need to ask why. In the case of embargo, the concept itself falls apart if it's super easy to continually pile tokens on piles, it breaks the game. That's a good answer. I'm not convinced that the answer “because I don't want there to be so much bidding” is a good answer for self-trashing. I also think this card gets muddled with adding curiou cards and also cards from the supply. It would be more focused if it were just curio cards. You could even make less of them, that would limit the amount of bidding, too.
The VP muddles the card – now we're bidding on the card AND vp? Why both?
I can't comment on every single one of the curio cards for time reasons, but I will point out a few fun ones: Curious Book is quite an interesting card. Curious Doll is a cool interaction built into it for a VP that I think you could iterate on as a great card all on its own. Curious Plans also sounds like a great expansion card. I think you had some really fun player interaction concepts in the Curio deck. I encourage you refine some of those as individual cards, without the curio deck concept.
This scales unevenly with multiple players. The solution is to say “the player to your left.” Now it's always the same power level, and you could test the balance more effectively. The issue with +buy cards is that they become automatic purchases, just because of the non-terminal +buy at the start of next turn. However, I think the +buy could be a more interesting part of this card if it triggers on cards that cost 4 or more—why do I suggest this? Because it takes a long time to ramp up to using a +buy to buy two 5 cost cards. But often in the early game you do buy multiple 3-4 cost cards. This would make the +buy have negative synergy with the duration effect, and make it more focused and interesting. It would probably increase the strength of this card, and you might have to cost it 4 – hey look, more synergy!
On the other hand, by making it 5, you now force the opponents to make the decision, oh, should I buy this shiny 5 I really want even if it gives my opponent +1 money? That's a more interesting decision than it would be with the change I proposed, though I think the card is less focused with it's +buys and the at least 5 cost. I really like this card, it just fell short of top 5.
Phantom Village
+1 Card
+5 Actions
The player on your left names an Action. For the rest of this turn, when you play a copy of the named card, discard a card.
5 actions … that's a lot of actions. But it's not even that helpful because for an engine to work, you need density of villagers rather than just one mega village. The discard effect is very interesting, and it encourages you to have a strategy that depends on multiple card names to work. I do like that! I just think the mega village attached to that makes very little sense. This card makes more sense if it was a drawing card, like a library, or a large smithy (starts out with +4 or +5) or a cheap laboratory. Honestly a cheap laboratory would probably fit well. Then the card becomes all about one thing – increasing or decreasing hand size.
Delegate is an interesting card, the more that it is bought by players in the game, the worse it is. So, my opponent bought a delegate. Now I've given them free access to the best action card in my hand. But if I buy enough delegates, then the best card in my hand will be a delegate – which they can't play. So it's worse for them. But then they buy a lot of delegates, oh dear now it's worse for me!
I really love that thinking, and I think the fallback of +2 cards is super necessary, otherwise this isn't that great. I think often this can be used as pseudo-villages. They are a nice equalizer: if my turn duds with no village, well, I won't dud unless my opponent also duds.
I don't know why the card is set-aside, since you immediately return it after playing. Why not, play that card from the owner's hand, leaving it there?
This is in the top 5 
Alliance definitely has some nice interaction with other players. The focus of this card seems to be improving this turn, however +3 cards in the middle of the turn is a lot worse than +3 cards at the beginning of a turn. So your opponents benefit much more from this. And, it becomes hard to actualy use the +3 cards because you don't get an extra action. I think this event needs to provide an action with the card as well, the cost of 3 is enough of a penalty. And then, you should probably add a (once a turn) to this, just for simplicity sake. I feel this card would be most useful in turns where you dud, oh crap, if I just had 2 more cards I'd get to my village! I appreciate those designs, because dudding is not fun.
Antiquarian
Type: Event
Cost: $2P
Once per turn:
+1 Buy.
Choose one:
Trash up to two cards from your hand,
or gain up to two cards from the trash, setting them aside and put them into your hand at the start of your next turn.
I can always pay 2P to get $2 coppers at the start of my next turn. You've said that this costs 2P “for obvious reasons.” The reason I imagine is that you don't want players to trash in the beginning. But I think that would be fine. Bonfire allows you to trash cards that are in play, it's cheaper and WAY better at trashing coppers. If you want to trash coppers with this, well, now the card effectively costs 2 more. I don't think you need to delay this with a potion. Perhaps I am missing another obvious reason. I don't understand why this provides a +buy. That seems just a little mixed up. Other events that do similar things (this is kinda like bonfire meets save/expedition) do not provide a +buy. I think you should reconsider this, as it suddenly gets a lot cheaper if it comes with a +buy, and not in a good way.
This card is very light on player interaction for this contest. It could sort of act like a lurker, but that seems far too bad leaving two good cards in the trash. It's more likely going to be used as a remodeler aide, and that isn't very player-interactiony.
+1 Card
+1 Action
+1 Buy
If the type with the most cards in the trash is:
Treasure, trash this for +$3;
Victory, trash this for +2VP.
Another card that cares about the trash. What's interesting is Shaman makes itself worse. It trashes itself if actions aren't the most common – which makes actions become the most common. In that case, you just have a market, not bad for $3. I think this doesn't create much player interaction, because you really need to trash coppers and estates – it's too good. And worrying about the order you trash in is probably worse than just trashing the right thing. I think in most games this Is going to be +1card +1action +$3 +1buy trash this, which is an interesting card, though probably slightly better priced at 4. But again, this is pretty low on the interaction side.
Sponsor
cost $3 - Action
+$2
Each other player reveals a Treasure from their hand (or reveals they can't). You may gain a copy of that card. If you do, they do so, too.
I love the concept of this card. I might want to reveal my silver so I can gain a copy of it; however, in most games, you're not trying to gain tons of silvers. So this card is unlikely to be useful until mid-late game when people are getting golds. So I don't see much value in this. Why isn't this card a treasure? It would have a nice synergy with itself. The card is about duplicating treasures, so this card should be duplicable, and it won't be crazy powerful if you did it like that (although it would have to cost more than 3. Beefing it to somehow get to 5 --maybe with some additional benefit--would be the best). The major problem with this card is it is political. Players in turn order A,B,C. Player C likes Player B more than A. When Player A plays Sponsor, C shows a copper. When player B plays Sponsor, C shows a gold. Because of this favoritism, Player B now does the same thing for C. Politics.
Gatekeeper (Action) [$4]
+5 Cards
Reveal your hand. The player to your left names a card from your hand.
When you play a copy of that card this turn, ignore its instructions.
This is similar in concept to phantom village – well, I like this concept, and as you read from my criticism of phantom village, I think this type of card works best with a drawer. The penalty in this case is a lot steeper-- it turns those actions into confusions and that's sort of like a discard (well, I think some duration cards still work the next turn? I am unsure. Anyone know?). Also, what does it mean to ignore instructions of a copper? I assume it means it gives you no $, but I think that makes this card quite weak. In the beginning of the game, it'll cripple your economy entirely. So it's only useful in the mid-game, which is when your opponent is less likely to know what is in your hand and has to think strategically. I like this style of thinking, but I do think “ignoring it's instructions” is a strong penalty, one that is difficult to reason about and ancitipate.
...
They are also a new type; secret cards. Secret cards have a randomizer and do not count towards the 10 cards in the kingdom (choose them in a similar way to sideways cards but you can play with all 3 of them). There are 1 copy of each secret card per player. Each player starts with their secret cards (however many they are playing with that game) upside down underneath their deck (they are not part of your deck, this is for reminder purposes only). The first time each player shuffles, they put their secret cards onto their tavern mat.
Every time I had a question on this, I reread it and realized it was answered. I love that this card is a sensible card that is neither action, treasure, night, or victory. Well done! I like the thought, when do I call this card? Do I wait till I have multiple? But I imagine many times, these +1s are far too good (specifically Library and village) at saving your turn, that you probably just call them when you need them, rather than strategically calling them. So I think the player interaction is a little more limited than it appears. Still. It's pretty fun to have those cards cycle around.
This is in the top 5
Climber
Action/Duration - $3
Until your next turn, at the start of every player's buy phase, if another player has more cards in hand than you, draw until you have the same number of cards in hand.
At the start of your next turn, +1 Card.
So, without militias this means I get a draw up to 5 at the start of every buy phase. Hm, That seems fun. Then there is a nice anti-synergy where climber draws you up to 6, so then other players who play climbers draw up to 6. Getting the benefit twice is nice, but I am unsure how helpful it is. Think about it this way – in order for climber to work well, you want to draw treasures with it (since anything else is drawn dead). So, you want to have a treasure-heavy deck. However, if you have a treasure heavy deck, you are far more likely to have at least 5 cards at the end of your turn and therefore climber will do very little for you. I am unsure how to fix this fundamental problem.
Polymath
Action, $3
The player to your left selects a non-Duration Action card from the Supply that you haven't played this turn. Play that, leaving it there.
I was really intrigued by this card, as it's a real awesome type of player interaction that I've explored in my own designs. I found that cards like this either need a fall-back or some other benefit (like +1 action) to work. Notice how delegate provides some additional benefit (if it misses, it gives +2 cards). I think cards like this need some small extra benefit to be more interesting (even if it's an on-gain benefit). I wanted this to be good, but I looked at 5 of the last Kingdom of the week on the dominion subreddits, and only 1 of those I would even entertain buying this. And I probably wouldn't even buy it then. It's too often incredible weak, and, it's pretty difficult to make it strong-- you have to play a lot of good cards in order to play lots of actions, and in that case, you've played lots of actions but there aren't many good ones left for polymath.
8/19 2:12pm
Very straightforward simple interaction. This is an event you probably buy multiple times in the game. Your opponent needs to figure out what cards you want, what cards you don't want. And the card itself encourages variety. You probably would never get a scout ever, but what if it had a +1 card token on it? Not bad! So, opponents have to worry about selecting a cheap cantrip (I don't want you to have a 2 cost pearl diver lab!). A thoughtful opponent may choose a different, “better” card that isn't a cantrip to avoid you from turning it into a lab, forcing you to buy the card a second time. And you, you might plan on buying this card twice, so you don't waste your +1 card or +1 action on a terminal silver card, but you wait to place those tokens on your second or third discover buy when your opponent has to name smithy or Caravan. I love the simplicity of this and how it forces your opponent to think through your interactions. This is an event that is more fun for the person to the left of the buyer. That isn't a critique, I think that's a positive trait.
This is in the top 5 
A couple of people have mentioned the swinginess inherit in this. I will add that getting rid of good cards is hardly ever going to be worth 2 coffers or 2vp. Also, you never answered my question, how is the or decided? Who chooses? The player playing swamp tower or the player who wins the highest in cost? The debt also is very attack-y. It's a much stronger potential attack than “tribute” or “masquerade” is and so it feels pretty bad to lose a copper, gain a estate, and get saddled with debt. I do appreciate that this card offers some unique sifting that helps the player of this card not get hit by the attack – I think that works. This card would be more focused if you only gave a reward or only gave a penalty, I believe. I do like the masquerade interaction and the encouragement to pass good cards.
Mason's Lodge • $5 • Action - Duration
+1 Buy
Until the end of your next turn, cards cost $1 less (to a minimum of $0) for all players, and all players get +1 Buy at the start of their turn.
Whenever a player gains a card after their first, you get +1 Coffers.
Wow, very interesting. You sorta give each other player 2$ if they gain two cards. It's hard to turn that down, but then you get +1 coffers. And you get a +buy to deal with all those coffers. Very tight. This card's power (without gaining coffers) is similar to merchant ship. If you buy two things then it's actually stronger than merchant ship. And then you can gain coffers. However that comes at a price, essentially playing a merchant ship for your opponents. One piece of feedback in dominion the way it is phrased is “, but not less than 0” (bridge, highway, quary, bridge troll, etc). Not sure why you phrased it as “(to a minimum of $0).” This was really close to the top 5.
Top 5:
Commune, Bartender, Delegate, Secrets, Discover.
All of these are really wonderful and simple ways that would help make Dominion feel less like solitaire. I'd very happy with any of these in a Kingdom
I decided to go with how much they impact the strategy of the game in terms of thinking in your opponent's shoes and adapting to them.
Commune and Delegate don't work too strongly here, Commune you are more likely to do a simple 1,2,3 decision and it's probably most aimed in yourself. Delegate you just try to show your worst card, not much thought goes into that. Secrets is more about, whether you play it or not, it doesn't change based on your opponent's strategy more, though I love how it impacts the game.
Bartender and Discover both have wonderful strategic implications, but to me, Discover is slightly more thoughtful. Plus I'm inspired by how simple it is.
Winner: Discover by pubby.
Runner up: Bartender by fly-eagles-fly
