In this blog I will adress several Castlevania games and related media, and explain if they either take place in IGA's offical timeline, are alternative continuities, or are set in completely different universes. All these works have been released during the period of 2007-2012.
Castlevania: Order of Shadows and The Arcade
For Order of Shadows and the Arcade, we proceeded from the start with the plan to create them as a separate continuity (gaiden), since we thought including them in the formal series would cause a variety of production difficulties.
IGA says these games are gaiden. Going with the distinction that he made between gaiden and "alternate universe", it means that the stories of these games are set in alternative timelines. For example, a character from Order of Shadows, Giovanni, is said to be related to Rinaldo Gandolfi, a character from Lament of Innocence. Therefore, it can be assumed these two games share the same continuity. Currently, it is unknown how Castlevania Arcade relates to the offical timeline.
Akumajo Dracula: Lament of Innocence
I do not now much about this comic adaptation of Lament of Innocence because it has only been released for a Japanese cellphone service (which has recently been taken down). However, being supervised by IGA, and being merely an adaptation that expands on the story, it's safe to assume it's in continuity with IGA's offical timeline.
Akumajō Dracula X: Tsuioku no Yasōkyoku and Akumajo Dracula: Kabuchi no Tsuisoukyoku
This radio drama and novella are completely original stories that act as sequels to Symphony of the Night and Dawn of Sorrow. Being once again supervised by IGA, and being sequels to already existing games in the offical timeline, it seems clear that they have a place in continuity, and can be enjoyed as such.
Akumajo Dracula Pachislot Series
These Castlevania pachinko games have been puzzeling some fans for quite a while how exactly they fit into the storyline. They star Trevor Belmont and borrow the style from Curse of Darkness (Yami no Juin). The developer's blog sheds some light on the issue and states they are gaiden set during the time Curse of Darkness would have occured. The story of the first two Pachislots are actually one story and are set in this alternative timeline. The third and most recent entry takes place before the other two and adapts the story of Castlevania 3 (Akumajo Densetsu).
Here is an overview:
Yami no Juin / Akumajo Densetsu/Pachislot III---> \ Pachislot Akumajo Dracula I/II
Castlevania: The Adventure Rebirth
This game is a reimagining of Castlevania: The Adventure.
Because Castlevania: the Adventure Rebirth is more a completely new game than a remake, I think it is best to treat is as a separate continuity (gaiden).
The intro shows flashbacks from Castlevania 3. Therefore, The Adventure Rebirth is an alternative timeline sequel to that game.
Castlevania: Lords of Shadow
Lords of Shadow is a reboot the Castlevania series and is set it a completely different universe than the other games according to the producer. This one is very straightforward.
Castlevania Judgment and Harmony of Despair
These two games bring characters from all eras of the Castlevania timeline together in one big inter-series crossover. In Judgment by the means of time travel, and in Harmony of Despair thanks to the magic of an ancient tome called the Grimoire. Some fans these two games fall completely outside of continuity, meaning that they are not related to the offical timeline in any way.
In case of Judgment, it's because Cornell is included in the game. He is a character from one of the KCEK gaiden titles Legacy of Darkness (Legend of Cornell). The reasoning of those believe Judgement is not in continuity is because Cornell is a non-continuity character, therefore the game is also this. However, they are not aware Cornell is from an alternative timeline, therefore still included in the continuity in some form, and that Judgment is set in a "time rift" that compresses time, which would explain Cornell's presence along side other characters from the offical timeline.
The reason why some fans think Harmony of Despair is not in continuity is because they think the game has no storyline. While it is true that the story is not explained in the game, this is not correct. In an interview, IGA explains the storyline and says that the setting of the game is within a magical book which recorded characters and places from the Castlevania series. The characters that the player controls are actually individuals who got trapped inside the book and try to escape. They attempt to do this by "borrowing" the powers of the heroes who are recorded in the book. There are no continuity issues in this game, so it is also in continuity.