Demon Castle War

The Demon Castle War (Also known as the Battle of 1999) is first mentioned in Castlevania: Aria of Sorrow, and occurred in 1999. In the game, you can acquire and equip an actual uniform from the battle. Also, some enemies Soma Cruz encounters in Dracula's Castle - the Zombie Soldier and Zombie Officer - are the reanimated corpses of soldiers who died some 36 years before (indicating the date of the War, as the game is set in 2035). It remains unknown who these soldiers fought for during the War, although it is possible they opposed Dracula's forces.

The specifics of the war are never directly explained in the game, but it is likely that it involves the events that lead Julius Belmont to defeat Dracula once and for all. Whether or not the exact details will ever be known remains a mystery, the other known main combatant in this conflict is Alucard (Adrian F. Tepes).

There is also no game in the Castlevania series which depicts or explains the events of the Demon Castle War at this time!

Fan Fiction
A fanfic is being written, currently on hiatus, titled "Castlevania: Genesis of Sorrow" from the perspective of one of the author's OCs, who joins Julius Belmont in the Demon Castle War. Said OC writes a novel of the event, and dies some time later. The fanfic's plotline shows the OC's younger brother finding the book and reading it with his wife.