Hm... I've completed 14 adventures and am not being offered any new ones, but the achievement states that I have to complete 16? Where are the other two?
I'd like to recommend some support for the color blind among us: The symbols on the pieces are good to differentiate on the board, but for the color wheel and the enemies it comes down to color only. Any way you could add the symbols from the pieces on the wheel and next to the enemies?
 "Dangerous adventure"){kind=icon}