The way the game works is that 100 players go through the same level of Super Mario Bros. Each individual has their own instance of the game and for the most part don't interact directly with the other players, which makes it appear that there are 99 other Mario "ghosts."
Each player can, however, interact with the level and in turn, affect the other players' instance of the level. If one player grabs the mushroom to make Mario big, that item isn't available to everyone else. A player could also jump on a Koopa turtle's shell, causing it to knock into other players and kill them. Things get even more interesting when one player obtains the star and goes invincible. Now, that player is active in everyone's instance and can eliminate anyone they touch.
There are several different worlds based on levels from Super Mario Bros. Each world has four levels, and the player who reaches the last level and gets past Bowser first will be the winner.
Mario Royale is available to play via a web browser, and players can use a keyboard or gamepad to control Mario.