Castlevania was a smash hit. Despite the difficulty of the game and its lack of quality in some areas it was loved worldwide and proved very popular. It was a no-brainer for Konami to create a sequel.
Perhaps borrowing ideas from Nintendo (Super Mario Bros. 2, Zelda II: The Adventure of Link, etc.) the game is very different from its predecessor. You don't venture through straight-forward levels until you get to the end; instead, there is a large overworld with many towns and dungeons ("mansions" in this game) scattered all around.
The plot is that Dracula, right before he died in Castlevania, placed a curse on Simon that would kill him. Obviously not wanting to die, Simon sets out to resurrect and then re-kill Dracula by collecting his body parts (collecting Dracula's body parts would pop up again in some later games in the series).
To do this he needs the help of various villagers. Some give helpful tips, others sell Simon weapon upgrades, Holy Water to throw, or other items. This is where a significant flaw becomes apparent: the text was written by somebody with seemingly no grasp of the English language, meaning some villagers that give necessary hints spew complete nonsense. This makes the experience extremely confusing and hard to figure out at times.
When you are inside towns or travelling outside killing monsters, every five minutes or so a box will pop up in the air which tells you that it's becoming day/night (during this you can do nothing for about 10 seconds). During night-time monsters are stronger, houses/shops are closed and enemies even appear in towns. Killing monsters will eventually level up Simon, making him more resistant to damage and giving him more life, while these beasts also drop hearts with which items can be bought.
In mansions there are some stronger monsters, while your objective is to find a salesman somewhere who will sell you an Oak Stake. With this, you have to go to the end of the mansion to find a glowing orb — throw the Stake at this to break it open and find one of Dracula's body parts. Only two of the five mansions actually have bosses, although they can just be walked past. Defeating them gains you a special item, but this is unnecessary. An annoying part of mansions, meanwhile, are invisible pits - it is impossible to discern normal floors from blocks you can fall through. Sometimes this means you will have to constantly throw Holy Water to make it through a whole mansion (as the bottles will fall through fake floors). Some jumps in the game also require you to be pixel-perfect, or else you'll fall down a floor and have to backtrack, or worse, you die.
After collecting all of the body parts you must go to Dracula's castle which, strangely, just consists of empty hallways with no enemies at all. When you reach the end, Simon will use the bodyparts and Dracula will appear. Disappointingly, Dracula is very easy and can be killed in no time with a certain trick, which will make him never get a hit in; after this, the ending is shown. There are better endings if you took fewer days to kill Dracula.
One thing worth praising is the music: this game has a fantastic soundtrack and formally introduces one of the now most frequently used Castlevania tunes - Bloody Tears. It was used before in the arcade game Haunted Castle, but not many people have played that particular machine.
Castlevania II, while introducing some neat ideas, ultimately doesn't feel that much different from other classic Castlevania games, other than the use of items and the overworld (which has some ideas that would be reused in "Metroidvania" Castlevania games). The game is relatively easy, possibly the easiest Castlevania game, though if you lose all lives, you lose all your hearts (which act as money). This can be frustrating if you do die frequently.
The game would be above average if it wasn't for the horrible, horrible script - you honestly have no idea where to go at times as villagers tell you utter nonsense. On top of that, the broken-English speaking villagers that are supposed to give good hints are mingled with villagers that are supposed to give bad hints, making it quite frustrating to separate the two types. Ultimately, while it's not as bad as a lot of people say, it does have a lot of glaring flaws that make it less than enjoyable.