I worked as a game programmer, mostly on UX and player vs player features.
The asynchronous dueling was an interesting mechanic, based on hit chance. I learned a lot about how biased players are when looking at odds for success.
The feeling of a 80% chance to win is emotionally very different from the actual odds.
I also worked in the sound implementation of the game, it had some interesting nuance. We separated the environment audio by prefab, and the closer you got to a saloon the louder the piano got compared to other sources.
It's all very standard now, but it was a bit groundbreaking (and maybe overkill) for a facebook social game.