The fifth console generation was a transitional period for video games. Even though there were experiments with 3D during the fourth generation, the fifth was when sprites were getting replaced by polygons all over the place. As a result, many established franchises had to adapt to the new 3D paradigm. Some did wonderfully like Mario and Zelda; some failed like Contra and Bubsy. Castlevania 64 fell in the middle of that spectrum when it released, but Konami rectified that for the most part with Castlevania: Legacy of Darkness.
Castlevania: Legacy of Darkness is a prequel to Castlevania 64. The main plot focuses on a werewolf named Cornell out to save his sister from being sacrificed to resurrect Dracula. The plot is fine though, like with Alien Trilogy, it becomes almost superfluous during the game. The ending (which I won't spoil) leads right into Carrie and Reinhardt's tale which, thankfully, you don't have to buy the prior game to play; their quest (with a couple slight omissions) is present on this cartridge, making grabbing Castlevania 64 completely unnecessary.
The two N64 Castlevania games try to marry the simple platforming of the older 2D installments with the heavier story focus of Symphony of the Night on the Playstation while moving into a new 3D world. The combination worked somewhat as back-tracking through the large open levels is limited. However, the areas lack a lot of detail and can become redundant. There were times I found myself running in circles because too many areas look the same. The combat is relatively simple and enjoyable but balanced more in favor of the characters with projectiles; Reinhardt with his whip is at a disadvantage in the game.
Konami had learned a few things from the first 3D Castlevania and applied what they learned here. The lock-on for combat is tighter; that's a good thing since the auto-targeting has issues with picking targets when more than one are nearby. The stages taken from the first game were streamlined to make them easier to navigate. The voice acting was removed in order to fit the additional storylines and stages onto the cartridge. The game does support the RAM expansion pack, but bringing the visuals up to high-res cuts the framerate in half; still playable but unappealing. Finally, the infamously terrible camera from the first game was adjusted significantly. Even though there are still moments where unexpected camera movement can kill, those moments are much fewer than in the previous game. These adjustments are also present in Carrie and Reinhardt's quest, making Castlevania 64 even more unneeded.
Castlevania: Legacy of Darkness is what Castlevania 64 should have been. The additional storylines expand on the lore well, and the gameplay fixes make even going through the older quest much more enjoyable. While later games in the series would do 3D much better, this one is still an entertaining romp. Bury Castlevania 64 in a shallow grave, and grab this one instead.