2.5D Game Play
What Is It?
In the age of the NES, all games were "2D". This didn't refer to the flatness of the TV (what with the modern innovation of actual, 3D TVs), but to the graphics themselves. All sprites were 2D art, so all games using sprites were 2D games. These games could either be top-down (such as The Legend of Zelda), 3/4 isometric view (Equinox), or straight on (platformers like Castlevania, shooters like Gradius, puzzlers like Tetris, etc.).
This didn't really change until the innovation of basic 3D graphics. Although some games on the SNES sported pre-rendered polygonal graphics (3D graphics) set as sprites on the 2D plane (as popularized by the Donkey Kong Country series), the first "true" 2.5D game is commonly credited as Yoshi's Story for the Nintendo 64. That game featured lush, actively rendered polygonal graphics set against 2D platforming game play. From there, the term gained widespread use for any polygonal game featuring a 2D playing field.
What Castlevania Games Are 2.5D?
When it comes to two-and-a-half dimensions, two games in the Castlevania series could take on the designation of being "2.5D games". The first is Castlevania: The Dracula X Chronicles, which was released in 2007. The game featured an enhanced remake of series favorite Castlevania Dracula X: Rondo of Blood, with polygonal graphics but classic game play. (some cut-scenes were rendered in fully 3D, though).
Then there was the entry in the Lords of Shadow sub-series: Castlevania: Lords of Shadow - Mirror of Fate. This game is played from the side-scrolling perspective but is rendered in 3D models very much like the mainline titles in the sub-series. It's something of a bridge from the older games of the past series and rebooted entries in this newer part of the franchise.
Going further afield, the canceled title Castlevania: Resurrection looked to have a 2.5D style to its game play. Portions of the game seemed to play from the side-scrolling perspective, although at times the linear path would have rotated towards the player with heroine Sonia Belmont having free roaming abilities within these areas. Of course, the game was canceled so players never got to experience this hybrid dynamic.
And then there's the two mobile games more recently released in the series: the since-canceled Castlevania: Grimoire of Souls and the in development Castlevania: Moonlight Rhapsody, both of which featured 3D graphics set against the 2D side-scrolling plane. And that says nothing of any o the other games in the series that have been drawn primarily in sprites but sometimes used 3D effects to enhance the game play. Sometimes the distinction between 2D, 3D, and 2.5D can get pretty fuzzy.