Castlevania Legends is the last Castlevania game on the original Game Boy, and probably one of the last exclusive Game Boy games before the Game Boy Color was released.

Unlike the previous two games that starred Christopher Belmont, this game stars Sonia Belmont, which I believe might have been the first female Belmont we've seen in the series.
I think this game was supposed to be the first in the Castlevania timeline, but then when an official Castlevania timeline was made, this game was left out of it and considered non-canon. I believe Koji Igarashi has later come out and said that he does regret making that timeline, so if Konami ever discards it, I wouldn't mind it, since it only really restricts things.

Anyway, there isn't much of a story here, but I do like how each of the Game Boy Castlevania games inject more and more dialogue as the games go along. It adds an extra boost to the production values, which is nice to see.

Speaking of production values, this game looks good, but not that much better than Belmont's Revenge. Considering this game came out in 1997, I can't help but feel a bit disappointed, but I don't really care for it in the long run.
I will say that I did like the soundtrack. There were a few songs that I didn't like, but what I did like was solid. Check the OST out if you're curious.

As for the gameplay, not much has changed from the previous titles, when it comes to the basic controls.
However, sub-weapons are bit different. Instead of getting them through candles in the stages, you will now receive a new Soul Weapon after you beat a boss. You keep it through the rest of the game, and you can swap between the ones you have using the Select button. I think this is a pretty cool addition!
The Soul Weapons we do receive range from the ability to stop time, to healing, to a screen-nuke.

In addition, the game's level design is a bit different from other Classicvanias, as in there are multiple paths that you can find in the stages... but a lot of them will lead to dead-ends, and you'll find more of them as you go through the game.
However, it's not all bad, as with these paths, you can find the Collection Items, which are represented by classic Castlevania sub-weapons. If you collect all of these, you will receive the best ending, which is nice.

But because of these dead-ends, and how long this stages took, it made the game a bit longer than it should be, making the game feel padded out.

Overall, while Castlevania: Legends can feel padded out and the enemy placement sometimes isn't the best, the new ideas it brings make this one stand out from other Classicvanias, and I had a decent time with the game.