Which Xbox game was the first to sell one million copies on Xbox Live Arcade?

Castle Crashers was the first Xbox Live Arcade game to sell one million copies.

The beat ’em up was developed by The Behemoth and released for Xbox 360 in August 2008. It combined side-scrolling combat with cartoon visuals, cooperative multiplayer, character unlocks, and a deliberately humorous medieval setting.

Its success was important because Xbox Live Arcade demonstrated that downloadable console games could become major commercial releases rather than merely small experiments. Castle Crashers eventually passed one million sales on the service, a milestone that attracted attention across the downloadable-games market.

Braid and Limbo are also celebrated Xbox Live Arcade successes, but they did not hold this specific first-to-one-million distinction. Castle Crashers is the relevant answer for the Xbox 360-era milestone.