Mario Bros. is a stage that greatly resembles the arcade video game of the same name that appeared in Super Smash Bros. Brawl for the Wii. The stage has four levels of platforms, with eight platforms in all. In the bottom center of the stage is a POW Block that'll cause harm to your opponenents and the enemies that'll scuttle across the stage. Sidesteppers and Shellcreepers will walk across the stage a lot.
The director of the game noted that this stage is vastly different compared to the other stages as it'll be quite a challenge to defeat your adversaries. The best way to do so, however, is to grab one of the enemies and toss them at your opponents, thus sending them flying sideways rather than upwards.
This stage is based off of the game of the same name. It takes place on the sole location in the game and features some of the enemies present in the classic arcade video game that introduced Mario's younger brother Luigi, including Sidesteppers, Shellcreepers, and those annoying fireballs that would cross the screen in attempt to kill one of the brothers. Even the POW Block is present in the bottom center as it was in the video game.
Shellcreepers can arguably be considered the predecessors to the Koopa Troopa species found within almost every Mario video game. If anything, they best resemble the Koopas found in the original Super Mario Bros. Following Mario Bros. Shellcreepers didn't appear in any major Mario video games, as they were, as aforementioned, replaced by the Koopas. In the original game, they would simply walk slowly on the platforms. Once kicked from underneath, they would retract into their shell where Mario or Luigi could then defeat it.
Sidesteppers also were introduced in Mario Bros. Here, they would simply jump across the platforms. They proved to be more of a challenge then the Shellcreepers as jumping underneath them would have you wait until they land on the ground. Afterwards, you'd once again have to jump on them to defeat them. Surprisingly, Sidesteppers went on to play a role, albeit minor, in Mario & Luigi: Superstar Saga in the sewer portion of Bean Bean Castle.
Fireballs in Mario Bros. were a nuisance - one of the biggest of all! They once again appear in Brawl though serve no purpose, excluding the attacking you part! As Mario players know, fireballs went on to serve a major part of the Mario series as something the Mario Bros. could shoot from their hands if garbed in the correct clothing.
The POW Block was also introduced in this game, and likewise it is stationed in the same place that it was in in the original Mario Bros. In the title, all of the opponents on the screen would flip over if either of the bros. hit it. Some of the enemies, however, such as the crabs, wouldn't if they hadn't already been hit before as it takes two strikes to take them down.