Well I just got around to attempting a playtest but it seems your patch is broken. I tried it in both ZSNES and snes9x and I got garbled title screen graphics followed by a freeze on the overworld both times. You should probably look into this.
Alright, I gave it a playthrough. Here are some thoughts.
The puzzle right at the start feels completely unnecessary and there are a number of ways it can go wrong. You can take a hit and be unable to spin jump the block. You can hit the shell on the thwomp or piranha plant and be unable to hit the vine block. You could hit the vine block without realizing you were supposed to spin jump. All of these lead the player to throwing themselves off the level to try again. If you insist on keeping it in the level, put a reset door but my recommendation would be to just remove it and the pipe transfer and just have the path continue into the area where the pipe takes you.
The ON/OFF section in the beginning has a few very precise jumps on to one-tile wide blocks, some of them with podoboo danger. It's inconsistent with the other ON/OFF platforms which up to that point have given the player plenty of room to land safely. Consider expanding the one-tile jumps to two tiles.
There are a few low ceiling jumps to be made through the level. Notably, right at the cave entrance there's a note block above lava with a very low ceiling and a podoboo taking up the same space. I cleared the gap in one go but I can easily see it being a tough jump for some. Later, in the castle portion, there's a three-tile gap between two platforms in a low ceiling area again. I only made the jump because I was running and ducked before jumping. Maybe the jump is easier when small but seeing as the cave portion isn't too difficult it's very likely the player will be coming into the castle with a powerup. Consider raising the ceilings in both locations to make the jumps fairer.
The fireball generator in the cave doesn't feel like it does much. It's very easy to avoid the fireballs especially since there are few enemies after the midpoint and there are lots of fences to climb. It's very easy to avoid fireballs on fences when given ample room to move, which you have done. I'd recommend removing the fireball generator and simply using more creative enemy placements to add difficulty.
Looking in LM, you don't appear to give the player a powerup after the midpoint. If the player dies, they have to go all the way to the castle section and survive until the fire flower just for an extra hit. That seems a bit unfair. Yes, the player could go back down to the powerup below but it's an unnecessary detour and I doubt most players would think to go back for it. Put something for the player in-between the midpoint and the castle section.
There should probably be a little more threat to the player while climbing fences in the castle portion. A number of them have nothing but simple jumps to worry about.
Big Boo probably needs another Boo Laser to be any semblance of dangerous.
It might look like I have a lot of problems with the level but in reality a lot of my thoughts are relatively minor. Overall the level feels pretty good. I like the patterning in the walls in the castle portion. The cave is good but just needs a little more something in it to make it great. You're almost there but you just need to iron out a few more kinks before it's ready. Hope all this helps.