It was first published in the magazine Galaxy Science Fiction, in December 1959, and has since been re-published in two anthologies and at least twenty-four collections.
The inspectors determine that the citadel is absorbing the soldiers one by one for an unknown purpose, and fear that the game may secretly be an atomic bomb building to critical mass.
They play the board game while waiting with a bomb disposal expert for the last soldier to disappear, but find that the citadel is actually a therapeutic tool to build confidence in children.
He accumulates the most holdings but learns from his children that he has lost; the purpose, according to the instructions, is to give up as much stock and money as possible.
The story concludes with the children, who are unfamiliar with Monopoly, "learning the naturalness of surrendering their holdings"; one says "It's the best educational toy you ever brought home, Dad!"