Look its a plane, no its a bird, no its parallax backgrounds!
The game uses parallax backgrounds. In Castlevania, this can be seen in the opening forest scenes (Symphony of the night) or through various windows as you progress.
So many weapons, so many enemies
Offer various choices for weapons. In Castlevania, normally you will have the primary weapon (whip) and a secondary weapon which can be used once you gather hearts. The secondary weapons have generally been: holy water, throwing knife, throwing ax, throwing sword, and crosses. Each will have different effects, and some are more potent than others for certain enemies.
As the player progresses, they gain strength, life bars, and other abilities that make them far more powerful than they are in the beginning of the game. Standard RPG's usually implement this in the form of 'levels' however, in the Castlevania universe, we don't have levels, just upgrades to stats.
Update, Upgrade, Outdate
The player will attain better armor, weapons, and other items that will help in their quest. This sometimes includes special abilities in the Castlevania world such as the ability to morph into mist which allows the player to go through semi solid doors, the ability to double jump allowing them to reach higher obstacles, or the ability to slide, allowing them a movement attack.
I never even saw it coming
Secret rooms hidden in the levels. These rooms will often contain power up items, or life extenders. You may have a very hard time winning the game if you don't find at least some of these items.
The game's point of view moves to follow the player's character, ship, avatar, or representation as they move around in the world.
The game has some non-trivial physics in it that the player has to deal with.
Save the Princess
The game involves rescuing, saving, capturing, or stealing an object (the pod in Thrust).
You Require More Vespene Gas
The player must deal with completing the game or level with limited resources (excluding time).