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My main issues with the game is the very low drop rate from enemies which makes getting things as small as a potion turn into a long drawn out grind session depending on your luck, the fact that there isn't a shop where you buy stuff is a massive problem because the other metroidvania Castlevania games had that, but this one doesn't for some reason, and the bosses and even some of the enemies do way too much damage. Some of those issues made Circle Of The Moon a unenjoyable game depending on the area on my first playthrough.

I would say that the soundtrack is pretty great. Circle of the Moon is defiantly my least favorite of the GBA titles, but it has my favorite soundtrack of the bunch. Lots of remixes from other Castlevania games. Like "The Sinking Old Sanctuary" is from Bloodlines (The GBA version sounds much better in my opinion), and "Vampire Hunter" from... Well, pretty much every other game in the entire series. As for the gameplay, it is a little stiff feeling compared to the other metroidvania style Castlevania games. Not to the point were it is a bother, but I found it a tad noticeable. The extra collectible you forgot about was the heart upgrade btw. You do start the game moving slow as molasses, but you do get those nice dash boots pretty early on, so it isn't a huge complaint.
The DSS mechanic is pretty fun, it isn't as cool as the soul system introduced in Aria of Sorrow, but it was a step in the right direction. There is one major problem with it though. The cards are dropped by enemies, as you mentioned. The drop rate in this game is very, very low. Especially for the cards, and considering that only specific enemies drop them and the game never tells you which enemies have the cards... It leads into some problems. Mainly that you could be slashing away at random foes over and over for hours trying to find all the cards. And while some enemies like the candle things in certain boss rooms are a little more obvious, it still is annoying. Nowadays it isn't a big problem, with the internet and all. But back in the early 2000s, with that primitive internet and limited information, it was a whole lot harder.
Finally I want to talk about the design of the castle. The design of the castle in Circle of the Moon is pretty mid in my eyes. It's big... and feels a little empty... I dunno, something about the giant open areas of the castle feel like big open empty spaces at times. As for the different areas in the castle, they don't stand out that much and tend to be rather basic in design. The castle in the game isn't horrid, it just isn't that interesting.
Overall, Circle of the Moon is a decent game. It has its problems, but for the first metroidvania Castlevania game to not only be released on the GBA, but come after the masterpiece that is Symphony of the Night, it does its job. Not bad, not super good, just okay. Many of its ideas would just be done better in future titles. Well, maybe the thing about Symphony of the Night being the first metroidvania Castlevania game isn't exactly true. I mean, if you think back to Castlevania 2: Simon's Quest that is!
Favorites:
The Sinking Old Sanctuary
Clockwork
Aquarius
The Trick Manor
Vampire Hunter

Castlevania circle of the moons soundtrack is good. I honestly forgot most of it, but from what I remember, it's pretty good. The story is your basic Castlevania affair with someone who resurrected dracula and you have to stop them. The gameplay itself is great, but the difficulty is as Castlevania as ever. What I mean by that is that the enemies and bosses do so much damage that it's ridiculous. Another thing I dislike about the game is that the only collectibles are Health and magic. (There is a third one, but I forgot the name of it):/ You get upgrades after you defeat a boss... besides the dash boots because you find that in the beginning of the game. You can also get cards from enemies to use as attacks or as an shield. That or you can do that famous glitch to get all the cards at the beginning of the game. Overall, Castlevania circle of the moon is a good game with a good soundtrack.
Soundtrack: 6.5/10.
Game: 6.8/10.