Look, I'm not going to lie: Milon's Secret Castle isn't very good.
You may have fond memories of it, because you played it when you were six years old and video games were so new you just thought it was awesome that you could make things move around on the color television your family had bought a few months before after finally upgrading from black-and-white. But based on recent observations, I've come to realize six-year-olds have terrible taste in video games, which is why parents aren't keen to buy $40 3DS games when they could anesthetize their rugrats with free iPhone games instead. What I'm saying is that whatever your nostalgia tells you about Milon's Secret Castle is probably wrong.
However, the nice thing is that Milon's Secret Castle is ever-so-slightly better on 3DS than it was in its original NES incarnation. That's because on 3DS, you can exploit the hell out of the game with save states.
See, MSC hails from an early era of video games, before developers had really come around to believing in silly things like "intuitive play" and "fairness" and "good design." It's pretty much an exercise in confusion. Styled after contemporary arcade games, it features a shop and even hidden bonus rounds, so you can acquire new equipment and upgrades, but the whole affair is just maddeningly opaque.
The protagonist, Milon, is a little kid in pajamas who seems ill-suited to such a brutal experience. He wanders into a castle full of monsters and proceeds to shoot bubbles at every physical surface in sight in the desperate hope of uncovering money, power-ups, and the doors to advance forward. There's a slight element of nonlinearity to the game in the sense that you can wander back and forth through the castle, but it's not the good kind of nonlinearity where you feel like you're making progress and occasionally backtrack to solve an object puzzle you spotted early. It's the bad kind of nonlinearity, where you miss a single tiny hidden detail and roam aimlessly trying every trick you know on every inch of the game in a desperate attempt to find the single stupid secret you missed.
And even that wouldn't be entirely unforgivable if the game were a little more merciful, but "mercy" is not in MSC's vocabulary. Milon is weak, and his enemies aren't so much powerful as they are brutal and unfair. They spawn seemingly at random, careening into him at high speeds without warning, their movements demonstrating no discernible pattern for you to avoid. Even worse are the bosses, which pummel you with rapid attacks -- and poor Milon doesn't benefit from an instant of mercy invincibility. So good luck beating the game, even if you make it to the final boss.
That's where the save states come in. MSC does offer a continue feature (though like everything else about the game, it's hidden), but the further you get into the quest the more likely you are to die instantly to an unexpected attack from nowhere, or a vicious boss encounter, undermining much of your work to date. Save states let you record your progress at will in ways the developers never intended. But that's OK, because the developers were jerks, and who cares what they intended?
There's actually the germ of a pretty fun game here, but you'd need to do a pretty significant overhaul of the adventure to make it work. If you want a free-roaming adventure where you break open every block you can see for your 3DS, why not just pay a few bucks extra and pick up Steamworld Dig?