The game is all about making the best deck possible, whilst already playing it.
Ok, so. The shared area of the game is 16 stacks of cards. Each player has a deck, initially of ten cards. Each turn you draw five cards (shuffling their discards if needed), then you get to spend one action (used by white cards) and purchase one card from the shared stacks (which goes into the discard). The game ends when one specific pile (the highest scoring one) or three other piles run out.
The player with the most points in their deck at the end wins.
And that's it. Various of the card types give more draws, more actions, more purchasing power - or do strange things like trade one card in for another, totally destroy cards, steal cards from other players hands...
Popular strategies are to buy nothing except money, or to try and trim your deck down to a small number of powerful cards which you draw each turn. Or to continually play the witch, if she is available.
There are vast amounts of different combinations of available cards, since six piles are the same every game, but the other ten are chosen from 25 options (with the base game). Certain cards combine very well with others, favour different types of strategy, and so on.
Ok, it plays pretty similarly (duh). There are three important differences. First is that a lot more cards force you to make a decision during your turn. Second is that there are interactions which do not count as attacks. Third is that several 'use just this card over and over' strategies are cards which are also worth victory points. So it looks like more games will end due to pile exhaustion than province use.
The rules also clarify: The basic treasure piles are supposed to be inexhaustible. I don't think it has come up, but it's worth mentioning.
And ironworks, buying a grand hall for an action and a card, oh I drew a grand hall, that's an action and a card, I drew the ironworks, guess I'll buy another grand hall... can take a while to resolve. --Vitenka
I have, in fact only played Intrigue. Great Hall is teh awsum. I had repeated turns of Great Hall, Great Hall, Great Hall, Great Hall, Conspirator (+2), Conspirator (+4), Coppersmith, buy something ludicrous (say, Nobles). --CH
After playing some more - we don't like saboteur, we didn't play with the other evil one yet. The defence card was generally thought to be too fiddly to be worth using against torturer, and usually got hit by a tribute right afterwards anyway.
The defence card (Secret Chamber) isn't really defensive. I thought it wasn't much use as a reaction - it might depend on what the attack was, though, I only saw it against Torturer. But I thought it was good as its ordinary Action - effectively, turn any useless cards in your hand into Copper. --Rachael
For my own interest (I haven't played with Secret Chamber yet), let's list the attacks and how good Secret Chamber is against each of them: --AC
Bureaucrat: Very likely to mean you don't get card disadvantage, at the cost of moving your VP cards from this turn's hand to next turn's.
Militia: Arguable. Gives you more choice of what to have in your 3 card hand, or the chance to save till next turn something that will no longer work.
Spy: Leaves you pretty likely to not lose something unless you want to.
Thief: Excellent - you won't be losing gold to the thief any more.
Witch: No good as its reaction effect, but its active effect lets you get some use of curse cards.
Minion: Excellent - keep the two cards you most want to have with a new hand. Makes being Minioned often a good thing.
Saboteur: Well, you're still getting sabotaged, but you get to set up what gets sabotaged.
Swindler: Very effective reaction, especially if one cost slot has very little there. "Put a Gold on top - want to turn it into a Harem for me?"
Torturer: Comments apply as per Militia and Witch above.
Oh yes, Vitenka has one other comment: Intrigue introduces a lot more points where you think "Ok, do I play safe, or do I play risky and maybe get the benefit of the safe thing on top of something else, and maybe get nothing?" Having a tribute and another action in hand is a good example of this.
This does, unfortunately, slow the game down. --Vitenka
It does. This is my biggest problem with this expansion: it plays significantly more slowly than the base game. --AC
The second expansion is out: Seaside. Lots of merchant ships, pirates and smugglers and all the rest of it. The principal mechanical theme is affecting your next turn. Cards do this in different ways, but the most obvious is the new card type, "Action - Duration". These Action cards stay in your in-play area past the end of your turn, and then at the start of your next turn, they do something and then get discarded. Images of the cards are [here]. Notable cards:
Lighthouse: A pre-emptive Moat - it protects you from attacks for the round that it's in your play area.
Outpost: Take an extra turn (but only draw 3 cards for it... at least unless you use other Duration cards). The wording is harsh enough that nothing yet printed - not even Throne Room - can get you more than 2 turns in a row.
Treasure Map: If you manage to get two in your hand at once, put 4 Gold on top of your deck!
The third expansion is Alchemy. It's got 12 new Kingdom cards, as well as a new currency, Potions, which some of the cards can't be bought without. The designer has provided [a little more information]. [BGG page] where more information will be rabidly reported.
It is great fun, though a couple of the cards take perhaps a little too long to resolve. --Vitenka
In particular, Philosopher's Stone; but that should be far more playable on CH's computer version :) --AC
The fourth expansion is called Prosperity. It's a full-size expansion of ~25 Kingdom cards. Beyond that, nothing is yet (WikiNow) known. But of course it does already have [a BGG page]. (And as of WikiNow it has card images.)
It has been suggested that a final round (up to the start player) be played out, with virtual provinces, to even things up. Vitenka doesn't like this, since previous games have often been very close as they are, and the tension of 'should I end it or buy a duchy instead', though rare, is quite nice.
Oh, it's also already the official tie-break condition (playr who has played fewer turns wins in the event of a tie on VP) --Vitenka
It has been suggested that 'Moat' always be present, perhaps as an eleventh pile. Because some people really really really hate being attacked, and whilst it may not always be the right option, they should have the choice of being safe.