Vampire Killer reuses a lot of assets from its NES counterpart, and presentation wise, it looks quite nice, but this is all to lure you into a false sense of security. Real quick, you'll realize what makes Vampire Killer so different from Castlevania. Hearts serve two purposes here: one is for your sub-weapons (standard stuff), and secondly, currency, kind of like Castlevania II. You use these hearts to buy from merchants who are sometimes in plain sight or behind a breakable wall. You have to hit these merchants a few times before they sell you anything, and after you're done with them, you can keep hitting them, and they'll eventually die. Free points!!! Speaking of breakable walls, they're everywhere in this game, and hitting at walls is vital as they can hide chests that can be opened with regular keys you find scattered throughout the levels, but more importantly, you might also find white keys. These white keys are required to progress to the next level. Unlike NES Castlevania, there's no boss at the end of every level; instead, they're at the end of every third level. There are 18 levels in total, which means a lot of hitting at walls and hoping you find those white keys.
You start off with your classic whip, but from the previously mentioned merchants, you can replace your whip and have, for example, an axe or a knife as your main weapon. They don't consume hearts, so spam to your heart's content. Although if you have the cross or the axe, which acts like the cross in this game, make sure to catch it on the rebound or you'll lose it and you'll be stuck with your shitty whip. You might find a merchant selling a candle; this will highlight which walls are breakable. Don't matter though, since your entire inventory, besides hearts, resets after beating a boss.
So truth be told, I was actually having an okay time with this up until stage 17, where I managed to softlock myself. See this block I subtly highlighted (this screenshot is not from my playthrough; I took this from a longplay on YouTube)? I got here after picking up the white key, but for some reason this block wouldn't break, so I checked a longplay on YouTube, and it seems to only break with holy water? Which I didn't have, so I had to jump off a pit and restart the level. This also reset my hearts, so I had to grind hearts to buy the holy water. Turns out there's another white key in this level close to the exit, so I didn't actually need to do any of that. The fun doesn't end there; Dracula in level 18 has a completely unique second form compared to his NES counterpart. He has a jewel on the top of his head, and to hit it you have to jump up these small platforms and attack from the top. The best method of attack is the knife. Luckily, the very first merchant you see on this level sells them for uhhhh 90 hearts. I didn't exactly get that many hearts back after spending most of them on the previous level, so once again, I grinded for these fucking things. Here's a very useful hint: you'll occasionally find red and white bibles. White bibles make merchant items cheaper, while red ones make them more expensive, so don't accidentally pick one up while you already happen to be holding a white bible, completely screwing yourself over. Play with save states if you really want to play this.
You start off with your classic whip, but from the previously mentioned merchants, you can replace your whip and have, for example, an axe or a knife as your main weapon. They don't consume hearts, so spam to your heart's content. Although if you have the cross or the axe, which acts like the cross in this game, make sure to catch it on the rebound or you'll lose it and you'll be stuck with your shitty whip. You might find a merchant selling a candle; this will highlight which walls are breakable. Don't matter though, since your entire inventory, besides hearts, resets after beating a boss.
So truth be told, I was actually having an okay time with this up until stage 17, where I managed to softlock myself. See this block I subtly highlighted (this screenshot is not from my playthrough; I took this from a longplay on YouTube)? I got here after picking up the white key, but for some reason this block wouldn't break, so I checked a longplay on YouTube, and it seems to only break with holy water? Which I didn't have, so I had to jump off a pit and restart the level. This also reset my hearts, so I had to grind hearts to buy the holy water. Turns out there's another white key in this level close to the exit, so I didn't actually need to do any of that. The fun doesn't end there; Dracula in level 18 has a completely unique second form compared to his NES counterpart. He has a jewel on the top of his head, and to hit it you have to jump up these small platforms and attack from the top. The best method of attack is the knife. Luckily, the very first merchant you see on this level sells them for uhhhh 90 hearts. I didn't exactly get that many hearts back after spending most of them on the previous level, so once again, I grinded for these fucking things. Here's a very useful hint: you'll occasionally find red and white bibles. White bibles make merchant items cheaper, while red ones make them more expensive, so don't accidentally pick one up while you already happen to be holding a white bible, completely screwing yourself over. Play with save states if you really want to play this.