Super Mario Bros. Crossover is a Flash browser-based platforming game. The game's levels and graphics are all primarily duplicates of those found in Super Mario Bros.; each of the game's eight worlds feature four levels. In three of the levels, the goal is to reach a checkpoint to complete the level, while the final level has the player facing against Bowser in hopes of rescuing Princess Peach. Enemies such as Goombas, Koopa Troopas and Hammer Bros. will block the player's attempt to make it successfully through the level. As Mario, the player can jump on enemies to kill them, use Koopa shells to knock enemies over, and acquire powerups from special blocks that serve to boost health, shoot fireballs, or remain invincible for a short period of time. Besides completing the level, the player can also earn coins and points, attempting to achieve a high score.
Crossover varies this formula by adding in several additional main characters in addition to Mario that the player can play. In the same manner as with Super Mario Bros. 2, the player selects one of the additional characters at the start of each level. Each character, based on other classic Nintendo Entertainment System games, uses different attacks and movements that are related to their original game, as well as their own signature music. These characters are:
Link, based on The Legend of Zelda, uses a sword and boomerang to attack enemies. Collecting powerups increases the potency of the sword's attacks and the range of his boomerang. He is the only character with fundamental control changes from his original game, being given the ability to jump, which wasn't possible until Zelda 2.
Mega Man possesses an arm cannon which he can use to fire forward to destroy enemies, with additional powerups to increase his firepower, and has the ability to slide under small spaces. Within version 1.1, Mega Man's jump height is reduced but now has the ability to call in Rush, his robotic dog, to assist in high jumps.
Samus Aran, from the Metroid games, also has a gun she can fire and which gains power with powerups. Samus is also able to transform into her "morph ball" form, rolling through narrow spaces and planting bombs. As of Version 1.1, the player starts off playing as Samus in her Zero-Suit, and upon the capture of a mushroom, gains her Power Suit. In Version 1.2, Samus can pick up missiles when enemies are defeated. Each missile sprite icon gives Samus 2 missiles.
Simon Belmont, the protagonist of the early Castlevania games, has a whip and can throw axes at foes, the attack strength and number of axes on screen increasing with additional powerups. He has the ability to double jump, which he doesn't have in the original games. His jump was also modified to be easier to control than the original games but can be changed back in the options with Classic mode.
Bill Rizer, one of the playable characters in Contra use a gun to fire on enemies, which increases in power and spread with further powerups. He can shoot in 8 directions, even when jumping.
Ryu Hayabusa from Ninja Gaiden was added in the version 1.1 release. Ryu, as a ninja, is able to use a sword and throwing stars, and has the ability to climb and jump walls.
Sophia the 3rd, the tank vehicle from Blaster Master, was added in the version 1.2 release. Sophia's capabilities include hovering, clinging to walls and ceilings, and homing missiles, but is also the first character with a limited amount of ammunition and power, requiring the player to refill these capacities through collection from defeated foes.
Luigi, Mario's brother from the Super Mario Bros. series was introduced in version 2.0. Luigi uses his gameplay from Super Mario Bros. The Lost Levels, being able to jump higher but not run as fast as Mario. He is also more slippery than Mario which makes it harder to avoid enemies and pitfalls.
Bass, from Mega Man 10, was introduced in version 2.0 and has similar play to Mega Man, but his arm cannon acts differently, being able to shoot in 7 directions. Instead of a slide, Bass has a dash that is faster than Mega Man's but cannot slide under small spaces. Like Simon, he can double jump.
In the version 2.0 update, character skins were a new and unique feature that allowed all characters to look different and have new music along with them. Some skins are the same character but from a different game in their respective franchise, while others are from completely different franchises. The 3.0 update added additional character skins, special rendering modes to mimic other earlier computer gaming hardware, and levels inspired by the limited-release Super Mario Bros. Special.
Source: Wikipedia, "Super Mario Bros. Crossover", available under the CC-BY-SA License.