Playing Boss Monster requires you to juggle two competing priorities: the need to lure Heroes at a faster rate than your opponents, and the need to kill those Heroes before they reach your Boss. Players can build one room per turn, each with its own damage and treasure value. More attractive rooms tend to deal less damage, so a Boss who is too greedy can become inundated with deadly Heroes.
Players interact with each other by building rooms and playing Spells. Because different Heroes seek different treasure types, and rooms are built simultaneously (played face down, then revealed), this means that every "build phase" is a bidding war. Spells are instant-speed effects that can give players advantages or disrupt opponents.