Often regarded to be the hardest NES game, let alone the hardest game ever made.
Stage eight is the final stage in the game to feature flies, which when eaten allow the Battletoads to regain health. Stages nine through twelve offer no opportunities to gain back lost health.
The Dark Queen is vulnerable from behind. The player can set up a rear strike by observing where she is when she comes out of her "tornado mode." When she ceases "tornado mode," she always faces the center of the screen. (Thus, if she comes out of it on the left side of the screen she faces right, and if she comes out on the right side she faces left.)
All of the original artwork for this game depicted all three 'Toads as being the same shade of green. In the game itself, Rash and Pimple were the same shade of green and Zitz was tan. The 'Toads' colors would be switched around several times throughout the series before Rare finally settled on giving Rash green skin, Zitz viridian skin and Pimple brown skin.
There are four warps in the game that allow players to skip levels. All of these warps skip exactly two levels and are all confined to the first half of the game. After stage six, there are no warps.