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 powerful enemies, enemies gaining extra characteristics, faster enemies, and reduced attack and defense ability of the player. This mode has had different names in some games, such as Crazy Mode. In Castlevania Judgment, there is an even harder mode that is unlocked after the completion of Hard Mode: Brutal Mode.
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 Bats or Medusa Heads in awkward places. 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. You start Hard Mode with the character you possessed when you beat normal mode. The player takes more damage in this mode. Additional creatures have been added and others have been swapped with more powerful ones. There are some creatures that appear in this mode that do not appear in the normal game, such as floating skulls that move similar to Medusa Heads, but with much more erratic and unpredictable up and down motions.
Starting a new game with the secret name "OKUDA" at the password screen starts a new game with just Trevor already in Hard Mode without the need of beating the normal mode.
Appearance in Curse of Darkness
In Curse of Darkness, the player can unlock the Hard Mode (called Crazy Mode), as well the Trevor Mode (that allows the player to play as Trevor Belmont. It is unlocked by starting anew game with @CRAZY as the save name. In this mode, all enemies are 10x the level compared to Hector, while retaining the same experience given in Normal Mode, making the game much more challenging and harder to level up. The enemies AI is also slightly improved, as they can sometimes synchronize attacks in order to break Hector's guard. Similar to Lament of Innocence, finishing the game on this difficulty allows the player to purcharse the Mobius Brooch, which reduces the cost to the Innocent Devils skills to zero.