So yeah, I can see why many people liked this so much. The presentation is sublime with excellent spritework and a memorable OST, and gives you a huge amount of shit to toy around with. The OST is very varied and has a ton of great composition, and deserves praise for
an underground theme which not only doesn't suck, but is also really great. The game even pulls a huge twist at the midway point which must've been seen as something really fucking cool.
Though like EYE: Divine Cybermancy, the majority of that huge amount of shit is unbalanced to the point of being useless outside a miniscule amount of situations you can just slash your way through with a stronger weapon and less effort.
While finding all kinds of crazy shit is fun initially as you stumble upon all kinds of weird shit, it will end up clogging your inventory when looking for that one weapon or item. You also get tons of potions and other powerful one-use items, but you'll never use them, because A) SotN is a PS1 game played with a DS1 controller, so you'll end up crawling through more menus rather than having a hotbar to assign consumables to which is a massive bother, and B) there's nothing really forcing you to use them. All of that huge amount of shit would be more useful if you were playing SotN with a keyboard and had to deal with less pace-breaking menus, and if SotN weren't so goddamn
easy.
Some bosses only dealt 1 HP per hit when I had like 300-400 HP. Whether this was intentional or because I accidentally sequence broke something, I don't know. Maybe because I saved up enough money to buy diamond armor? The messed up difficulty means bosses aren't that frightening, but more of a battle of attrition thanks to the bloated HP of bosses. Most enemy types you face aren't all that hard and the room layouts are equally as boring. You get plenty of healing items to make up for any mistakes anyways, with a vendor selling infinite potions. How you use those healing items is equally as silly, as you have to pause, free one of your hands, put the item there, and then 'use' it. The effect of potions is strangely delayed, whereas food is thrown on the ground and has to be picked up so it can heal you, all of that rather than directly being able to use it in the menu. Richter mode seems to be more challenging, though.
I still don't get why IGAvanias still bother with a level system instead of just relying entirely on gear for progression. To name a better example, in Ys games, levels exist to make you fight regular enemies (aside from it being so fun) for XP in order to prevent the player from skipping everything, and when you get to the boss, the way how leveling works in Ys means that being underleveled and dealing fuck-all damage means you haven't killed enough mobs, or you could grind to deal with bosses more easily when you're having a hard time (but never to the point where you are steamrolling that boss). Better gear can be found through exploration(!) or looting enough money off of enemy corpses.
In SotN, the bonuses of each level are fairly incremental, and grinding one level before a boss isn't going to make a significant difference. In addition, the non-linear design of the castle makes it hard to balance around what level the player would be when facing a boss, unless you're going to resort to scaling. It's not like you can distribute stat points by yourself, so the potential for builds is pretty much non-existent. Doing away with levels and stats (except for maybe HP max upgrades to make backtracking easier) and focusing entirely on gear progression would have made things much easier to balance. It worked for Metroid. Later IGAvanias (and to some extent SotN) already take care of a reason for killing mobs by having them randomly drop useful shit like souls and cards on death, so I don't really see the point of leveling.
The huge amount of shit you fight on the ground can range from a simple sword, to a suit which heals when being attacked by cats, to boots which discreetly increase jump height by increasing sprite size, to a sword which doesn't do damage at all, to a fake weak set of items which provide a luck bonus when worn together, to a rod which provides a special effect depending on your equipped shield (with the endgame shield combo instakilling fucking everything), to a combat knife which is OP OP OP. Seriously, it's like KCET didn't even know about DPS and that attack speed is a thing too. All of this feels thrown into the game with little regard for balance, any idea some guy came up with was probably coded into the game within 15 minutes. Thankfully this design philosophy does carry over to enemy designs, so there's almost no pallet swap and enemies feel fresh, with some enemies appearing in only one room. Unfortunately the enemies are rarely properly utilized.
Combat was pretty different from the Classicvania, by allowing more control over your character, multiple weapon types with their own attacks, and so on. Subweapons are still there with some new (questionable) additions, most of which I never really used because I was already a bulldozer. There's a separate mana bar for all your combo and transformation needs, which doesn't really ever have to be managed unless you strongly rely on combos or transformations, which is fairly unlikely. You get shields, but they are only good for deflecting projectiles and do jack against direct hits, they don't even reduce damage when holding them (aside from a passive bonus). Backstepping lets you quickly dodge stuff, unfortunately canceling out of attacks into a backstep wasn't a thing back then yet, which could be useful for the heavier weapons. Combat against most enemies is okay for the most part, they weren't as HP-bloated here as they were in some later games. Though the enemy attacks weren't as varied as they were in CotM.
Each vampire also gets three transformations, The Wolf, The Bat, and The Smoke. Wolf lets you perform longer jumps, and later upgrades give you the ability to run through everything with invincibility á la the Super Metroid Speed Booster. But the layout prevents you from doing that non stop and each enemy you hit drains a portion of your MP. Could it be that this was actually balanced and intentional? I'm betting on sheer coincidence instead when priorities were probably different early on in development. You also get to fly around as a bat and explore everything else in the map. Later upgrades include a sonar which is only useful in one room, and a fireball which takes too long to wind up while dealing shit damage as well. Sometimes enemies are placed to disrupt your progress in Bat form as one hit knocks you back to being human again, but that doesn't happen often enough. Last one is smoke, which is used for about two-three times two gates in the game which can only be passed through with smoke. Anyways, you can slowly move around as smoke and not take damage at all, so I guess it'd be an extension to backstepping by rendering you invulnerable at the cost of MP. Unfortunately transforming into anything takes way too long, so evading attacks with mist is impractical as all hell unless you see attacks coming from a mile away, in which case you might as well move away. Later upgrades include being able to move around in smoke form, and damaging enemies who come into contact with you, which isn't as useful as one might think.
There's even fighting game-like inputs which allow you to execute special attacks, which you can all perform from the beginning, for some reason? Most of these inputs require diagonal inputs, and are usually one movement rather than LEFT RIGHT LEFT RIGHT PUNCH or something along those lines. It would've been easier to perform with an analog stick, but I don't know if SotN was made with analog sticks in mind, and if so I couldn't have done it with an analog stick because I'm playing SotN via DCX on the PSP and I HATE the PSP analog stick (unfortunately the DCX version replaces the cheesy voice acting too). These special attacks can range from a homing spirit, to performing Dracula's iconic fireball attack, to multiple homing spirits, special attacks for your transformations, and so on. Some inputs require you to hold a button for a second or three, which seems like a massive waste of time. You should be able to find these inputs through scrolls, some of which you can buy, although I've never really found one on the field, (maybe I didn't explore hard enough?) so thanks internet. This sounds cool on paper, but once again, isn't as practical in-game. I hear Richter Mode makes really cool use of these combos, though.
And to stuff even more content in the game, you have familiars, little flying helpers you can find in the castle and use them to aid you in your quest. They can also gain XP and level up (to do more damage?). The problem is that they don't really do much damage, and their attack intervals are so long to the point of being discardable, initially anyways. Though the XP required for the familiars to be useful seems to outweigh the length of a regular double castle playthrough, so it's unlikely you will level one familiar to a useful state in one playthrough unless you grind hard. But if you ask me, familiars are just another additional layer the devs added 'because it was cool'.
You can get the Bat form and kick Richter's ass immediately in a cool boss fight which is once again made laughable by the non-existent balance. Or you can explore some more for getting specs which allow you to free Richter from his control and move on to the upside-down castle. I don't know why an upside-down castle had to be summoned out of nowhere, but whatever, the point is that you get to do everything again but upside down. This time you have all your powers, and there's no barriers preventing you from exploring anything. And when you get there, enemies hit hard, more new enemies get introduced, more hazards get introduced too, and suddenly the game seems challenging. Well, that is, until you level up some more, get your Alucard gear back, and laugh as everything deals 1 damage again. Everything being upside-down is cool (although you have to rely too much on Bat form or Gravity boots to get around), but at that point it's easy to get bored with the game as it still is the same castle you went through but upside-down. The cool catchy music has changed to more boring dramatic stuff, which I didn't really like. You can skip directly to the Dracula boss fight, although that's not really recommended.
To me, SotN is more of a game about 'look at this crazy shit I can do' than a coherent experience designed around using the full extent of your abilities, like Metroid. SotN has too much abilities with questionable usefulness, and would be more suitable for an ARPG like Diablo or Path of Exile if you consider the huge amount of shit the game throws at you. The aforementioned games also have that crazy amount of content, but are more properly balanced around multiple playstyles, something which CotM would kind of try to do. AoS/DoS would dial back the huge amount of shit into something more refined and balanced for the better, while still allowing players to be creative in their use.
So yeah, it's an unbalanced mess, though it can still be considered a fun unbalanced mess. SotN doesn't do things really wrong, but the execution is really hit 'n miss. SotN is basically like someone threw a thousand darts at a dart board. Eventually some will hit a bullseye, but most darts will stick around the center. It didn't completely bore me to tears like HoD did, but did enough crazy stuff to keep me playing. There's a fuckton of secret effects to pretty much everything in the game, so secret hunters will definitely find more enjoyment out of this game. It's definitely the presentation and sense of wonder and exploration which carries the game, but sometimes the game can't really carry its own weight. As a whole, it's okay. Could've been done better, however.
friends and I used to create classes and races too(fully fledged with newly created abilities, spells, exp chart etc), albeit while keeping the Advanced D&D 2nd edition system, which is my fave ruleset of all time for tabletop. Best ones we did was Phantasy IV based universe 
with the appropriate races and classes etc. and interstellar travel. No joke! We were obsessed with that game back then, haha.
