The Time Rift (aka time gap) is where all the characters in Castlevania: Judgment are trapped in. In order to escape, one of them must defeat the Time Reaper. This phenomenon does not respect the normal passage of time, and so allows individuals from different time periods to be in it at the same time, such as:
Sypha Belnades from a time before Castlevania III: Dracula's Curse
Simon Belmont from a year after Castlevania
Maria Renard from two years before Symphony of the Night (three years after Rondo of Blood)
Shanoa from during Castlevania: Order of Ecclesia
Carmilla most likely before Circle of the Moon
Death from a time after Symphony of the Night and perhaps just before Legacy of Darkness
Cornell just before Castlevania: Legacy of Darkness
Alucard from a time after Symphony of the Night after the Alucard Spear had been passed to the Lecarde family, but before it passed to Eric Lecarde
Eric Lecarde from ten years before Castlevania: Bloodlines
Golem from an unknown period after Carmilla entered the timerift
Dracula from a time of his resurrection after Bloodlines, perhaps before the Battle of 1999 (but no earlier than Symphony of the Night, as he mentioned that the last time he fought Alucard, he wasn't at full strength, as he was prematurely awakened).
Time Reaper from 10,000 years in the future, the general time period of the Kid Dracula games
Against certain individuals, the Time Rift has side-effects, in Cornell's case, it locks him in beast form, in Golem's case, it gives him self-awareness, both returned to normal once they left the Time Rift back to their respective time periods.