If you liked Castlevania 1, you'll probably like this game. It's basically more of the same, though this time there's a lot more content. On top of the game just generally being longer and having more stages, there are also branching paths and even multiple endings to encourage replays.

In terms of difficulty, this game is somehow even more brutal than Castlevania 1. The levels are longer, and you only start with two extra lives instead of three. The boss fights are generally more difficult than in the first game, but I also find them to be a lot more fun. The actual level design is weaker (for some reason there seems to be a larger emphasis on platforming), but it still captures the rewarding skill-based gameplay of the first game. This is one of the hardest games I've ever played, but I still very much enjoyed my time with it.

It's worth noting that the famicom version of this game is slightly more forgiving, and it also features improved music and visuals due to the cartridge containing a special chip that wasn't compatible with the NES.