Complex is being used in a different context here though. You are meaning complex as in strategic complexity. The parent (I think) is using it in terms of the games rules. Go is pretty simple in principle: place stones on the board one at a time, and remove enemy pieces completely surrounded by your own. But it's fairly complex to know what exactly constitutes being surrounded. Chess has 6 different kinds of pieces that have different movements, and different kinds of rules depending on the state, and promotion at the end, castling, and en passant. Checkers is a much simpler game because there's only 2 kinds of pieces, and 2 kinds of moves. Strategically, Chess isn't leaps and bounds more complex than Checkers since Carlsen has an Elo rating of 2800 in Chess, and Tinsley had an Elo rating of 2700 in Checkers and I believe it's the same deviation of 200.

Yeah, "deep" is probably a better term than "complex" to describe Go.

By this measure, is there a more complex game than Go, but which is not as widely played?

(Question from someone who does not know anything about board games.)

I ask because a natural question arises: can one engineer a game specifically so that it is extremely difficult for a machine to master compared to a human -- due to some strategic complexity, or some approach that does not lend itself well for current AI models.

edit: looks like there's at least one http://arimaa.com/arimaa/

If the human race had collectively had only a decade of Go-playing experience (as was the case with Arimaa), it might have been possible for an AI coded with "classical techniques" to defeat them.

(To be specific, my guess is that we'd have been at the amateur-dan level, and the initial MCTS revolution would have been enough for the computers to win.)

Cones of Dunshire? :)