The Adventure ReBirth consists of six areas that the player has to complete in order to finish the game. Toward the end of each area is a boss the player has to defeat before advancing to the next stage.
The player's main weapon for attacking in-game enemies is a whip, which can upgraded by collecting orbs. Unlike the original Adventure title for the Game Boy, there are secondary weapons that are referred to as sub-weapons, which are required to use with items called hearts. There are five available sub-weapons and each have a different use.
Source: Wikipedia, "Castlevania: The Adventure ReBirth", available under the CC-BY-SA License.