Shuffling…
Two decks, ten columns and eight foundations. Columns build down in suit, one card moves at a time, and the stock turns once and never returns — spend it like money.
ObjectiveBuild eight foundations up from Ace to King — two of every suit — using all one hundred and four cards.
Forty Thieves deals forty cards face up in ten columns of four, and that opening wall gives the game its name. Two full decks are in play, so eight foundations must climb from Ace to King, two per suit, and a hundred and four cards stand between you and the end.
The rules allow very little: columns build down in the same suit, exactly one card moves at a time, and the stock turns one card with no redeal. Any card may start an empty column, which is the single genuinely generous rule in the set and the pivot the whole game turns on.

Most patiences let you carry a sequence in one gesture. Forty Thieves does not, and that single restriction is where its reputation comes from. Relocating a four-card run needs four separate landing places, found in the right order — so distance is a real cost here in a way it simply is not elsewhere.
Work it out once and the game makes sense: a Five three columns away from its Six is not one move, it is however many moves it takes to build scaffolding for each card in between. Players who lose at Forty Thieves usually lost twenty moves earlier, by starting a rearrangement they could not afford to finish.
With no group moves, a column is the only way to relocate a sequence — every rearrangement is paid for card by card, so a free column is where all your future problems get solved. The shortest column at the deal is the cheapest to clear.
Each turn is irreversible and the waste only grows. A card buried under thirty others is gone for the rest of the game, so a stock turn should answer a question you have already asked, not fill a silence.
An off-suit stack cannot be moved at all here, so it is not slow progress — it is a wall you built yourself. The tempo you appear to lose by waiting for the right suit is tempo you would have lost twice over later.
Two decks mean two Aces of Spades, and a buried Two costs you a foundation you cannot start. Knowing which copy is where converts the game's most common dead end into something you can plan around.
Deals are random here. Forty Thieves' difficulty is traditional and genuine, and only a minority of random deals can be won even with careful play — which is precisely why some sites quietly hand you unwinnable ones without saying so.
Unlimited undo does a great deal of work in a game this unforgiving, since most losses trace to a single irreversible stock turn. If you want the same two-deck scale with a guarantee behind it, four-suit Spider is built winnable by construction.