Hard Mode

Hard Mode first appeared in the very first Castlevania game made and most games have followed this tradition. It is usually playable after the game is beaten on Normal Mode. Often times, it features additional enemies in the area, enemies being replaced by more powerfull enemies, enemies gaining extra characteristics, faster enemies, and reduced attack and defense ability of the player.

Appearance in Castlevania I
In Castlevania I, Hard Mode is an additional quest that takes place after Simon beats Dracula in Normal Mode. As there are no saves or passwords in the original NES version of this game, it is not possible to play Hard Mode without first playing through the entire Normal Mode during that session. It features additional enemies, such as Batss or Medusa Heads in awkward place. Some of the enemies, such as the Zombies move faster. Each enemy deals twice the damage that they normally do, even in early levels. Additional hidden crowns which are worth many points can be found in this mode. Every time you beat Hard Mode, you can start another round of Hard Mode (I'm not sure the enemies get tougher each time you beat Hard Mode).

Appearance in Castlevania Adventure
Hard Mode returns in Castlevania Adventure. Upon beating the game in normal mode, you can play through it again on Hard Mode. The creatures are stronger in this mode (I'm not sure if the deal more damage or can take more hits or both). Every time you beat Hard Mode, you can play an even harder mode.

Appearance in Dracula's Curse
Hard Mode is found in Dracula's Curse after beating Dracula in Normal Mode. Because of the password system, this is the first game where you can start a new session in Hard Mode, provided that you beat the game before.