Castlevania 2 is very different from its predecessor. You don't progress through linear levels until you reach the end. Instead, there is a large overworld with many cities and dungeons or mansions scattered throughout.
The plot
Dracula cast a curse on Simon just before he died in Castlevania. Simon obviously doesn't want to die and sets about reviving and then killing Dracula again by collecting his body parts.
support of the villages
To do this, he needs the help of various villagers. Some give helpful tips, others sell Simon weapon upgrades and throwable holy water or other items.
The cities
When you're in cities or out of town slaying monsters, a box will appear in the air about every five minutes, telling you it's day or night. During this time, you can't do anything for about 10 seconds. During the night, monsters are stronger, houses and businesses are closed, and enemies even appear in cities. Killing monsters eventually levels up Simon, making him more resistant to damage and giving him more life, while these beasts also drop hearts that can be used to buy items.
The villas
There are some stronger monsters in mansions, while your goal is to find a seller somewhere who will sell you an oak pole. With that, you have to go to the end of the mansion to find a glowing orb - throw the stake at it to break it open and find one of Dracula's body parts. Only two of the five mansions actually have bosses, although you can just walk past them. Defeating them will give you a special item, but that's unnecessary. An annoying part of mansions, meanwhile, are invisible pits - it's impossible to tell regular floors from blocks you can fall through. Sometimes this means you have to keep throwing holy water to make it through an entire mansion as the bottles fall through fake floors. Some jumps in the game also require you to be pixel perfect, or you'll fall down a floor and have to backtrack, or worse, you die.
On to the castle
After collecting all body parts you have to go to Dracula's castle, which strangely consists only of empty corridors without any enemies. When you reach the end, Simon will use the body parts and Dracula will appear. Disappointingly, Dracula is very easy and can be killed in a jiffy with a certain trick, which will never land him a hit. Then the end is shown. There are better endings when it takes you fewer days to kill Dracula.
The sound
This game has an amazing soundtrack and officially introduces one of the now most used Castlevania tunes - Bloody Tears. It was previously used in the arcade game Haunted Castle.
Classic Castlevania
Ultimately, while Castlevania II introduces some nice ideas, it doesn't feel very different from other classic Castlevania games, apart from the use of items and the overworld, which contains some ideas that would be reused in "Metroidvania" Castlevania games. The game is relatively simple and possibly the simplest Castlevania game. But if you lose all lives, you lose all your hearts that serve as money. This can be frustrating if you die frequently.
Conclusion
The game would be above average if it weren't for the awful script - you have no idea where to go when villagers are telling you utter nonsense. Furthermore, the broken English-speaking villagers who are supposed to give good tips mix with villagers who are supposed to give bad tips, making it quite frustrating to separate the two guys. Ultimately, while it's not as bad as many people say, it does have many glaring flaws that make it less than enjoyable.