When we start talking about difficult in-depth I think we should give a description of what we mean by difficulty. In theory, no Castlevania game is difficult because there isn't ever a situation where you absolutely must take damage. It's all a matter of memorizing patterns and dodging accordingly. It's not like your typical RPG where it's trade and receive. A good player is guaranteed to never get hit.
Now, the chances of someone dodging every attack are slim. It's possible, and the chances increase with play (as you learn enemy patterns), but on average you'll be taking hits here and there. If you ramped the difficulty by having enemies deal more damage, this doesn't benefit new players.
Experienced players are more likely to play more carefully, but otherwise, the "difficulty" doesn't necessarily increase, you're simply taking more damage, and playing the game more cautiously to compensate.
On the other hand, if you change enemy AI patterns to be more aggressive or otherwise, the same idea applies. This simply takes a different level of memorization and perhaps more nimble fingers. But again, it's possible to dodge every attack.
When we talk about increasing difficulty in Castlevania, I think in actuality we're saying, "increase the chances and situations wherein a player will by statistics take damage". I don't really know what to think of that. Do modern Castlevania games really need this? I guess that's what we're discussing.
I should also note there was another aspect to this in the pre-RPG Castlevanias; character control and stiffness. This added another potential variable to the equation where the player may find themselves in a position where they'll take damage in some way.
My proposal is a combination of all of these to even out the difficulty;
*Change enemy AI patterns. Make them more aggressive and possibly have enemies attack in random patterns that can't be memorized.
*If the player is taking too little damage, increase the damage dealt by enemies. In some ways, the RPG elements are pointless and serve only to create imbalance in the difficulty formula.
*Either restrict player movement or increase enemy nimbleness (depending on the enemy).
*My own idea, depending on how much or how little damage the player is taking, the enemies may become more/less aggressive accordingly. That would require some competent programmers.