Castlevania: Circle of the Moon/Cards

The magic system Circle of the Moon (called "Dual Set-up System" or simply DSS) involves pairing magic cards in order to create an effect. Some pairs boost your stats, other modify your weapons, and others are more traditional magic spells. There are two types of cards: Action cards which determine the type of effect, and Attribute cards which determine the element associated with the effect. There are 10 of each type of cards, giving you 100 different effects (though many of them are the same thing, but with a different element).

Cards are randomly dropped by specific enemies, which means that you may have to fight 40 or 50 of the same enemy before you get the card you want. The easiest way to do this is to find a room (generally one of the many hidden rooms) near a save point where the only enemy in the room is the one you want. For example, the best place to get the Manticore card is from the Thunder Demon in the hidden room nearest the Iron Golem boss. It's the only enemy in the room so you don't have to fight through hordes of monsters just for your 1-in-40 chance of getting the card, and the room is small so you can exit and re-enter quickly. This makes the rather tedious process go by much quickly.

Action cards are named after Roman gods and goddesses which Attribute cards are named after mythical creatures.

Mercury
Somewhat useful, but not really, as most enemies aren't particularly affected by elemental attacks. It is generally more effective to use the Venus and Salamander cards instead to raise your base strength by 25%.

Venus
Very useful as it raising stats without taking into account elements. Six of the cards raise strength or defense - either by raising one by 25%, raising one based on how much of the map is complete, or by trading one for the other. The Venus and Manticore combination doubles the number of hearts received (so 2 for a small, 10 for a large), which expedites replenishing hearts after a boss fight. The Venus and Cockatrice combination gives you free experience from walking. Sounds good, but since you only gain 1 exp for every five steps or so, it will talk hours to raise your levels, rendering the card combo useless.

Jupiter
Several of these cards produce shields that, while they don't stop attacks, can damage enemies that get close. Others produce shields that block projectiles but not contact with an enemy. Jupiter and Thunderbird cuts all damage by 50%, and Jupiter and Mandragora can heal you (making it possibly the most useful combo in the game, given the rarity of healing items).

Mars
Similar to the Salamander card, but it replaces your whip with a different weapon; usually a sword. Notable combos are the Griffin (a charge attack), Thunderbird (fast attack), Unicorn (very powerful holy sword), and Black Dog (slow but very powerful gun).

Diana
Another elemental damage card, but gives you a long range projectile to throw at the enemy. Useful for enemies that are too far away to hit or bosses that you need to avoid at all costs, but the high MP cost of most of the weapons makes it something to save for special situations.

Apollo
Objects are chucked at enemies in a similar fashion as the axe, and most explode on impact. The exceptions are the Golem, Cockatrice, and Unicorn combos, which act more like Uranus summon monsters.

Neptune
Absorbs all damage from a certain element, and restores 10% of the damage (so an attack that would normally take 100HP from you will instead take 100 MP and restore 10HP to you). Very useful, however note that some enemies have non elemental attacks, and running into an enemy won't necessary count as elemental damage.

Saturn
A elemental spirit appears and follows you around, firing when you whip. Fun, but not exceptionally useful since you can't control the familiar and most don't move around much. They tend to stay a little above you, meaning that many of their attacks will pass right over an enemy.

Uranus
A large creature appears on screen and unleashes an powerful magic attack that hits everything on screen. High MP cost, but still very, very useful against bosses.

Pluto
These are special effects that don't really fall into a category. Some affect stats, others how your sub-weapon works. The most useful ones are Pluto and Mandragora (+20% experience from all monsters - keep this one active when you aren't using anything else and switch to it when a boss is about to die), Pluto and Unicorn (complete invincibility, though you cannot attack - good for when you need to flee to a save room to heal), and Pluto and Black Dog (turns you into a skeleton).

Salamander
The most notable use of this card is with Pluto, which lets you use Item Crushes.

Serpent
The most important use of this card is that some combinations let you freeze enemies like the Brain Floats, which is necessary to get certain items early in the game. Aside from that ice weapons are generally useless as most Serpent combinations results in significant strength penalties.

Mandragora
Jupiter and Mandragora can heal you. Get this combination as soon as possible as healing items are rare and save points can be far apart. Pluto and Mandragora gives you a 20% experience bonus for all enemies. When you aren't using another combination, keep this one on at all times.

Golem
The big use is the Jupiter combo which makes your invincibility after getting hit last four times as long. Besides that, the card isn't that great, though Venus and Golem can be useful late in the game.

Cockatrice
Similar to Serpent combos, most Cockatrice combos results in the enemies being petrified. The big difference is that the combos generally give strength boosts as opposed to penalties. Still, most of the combinations are fairly useless as few enemies are weak against Earth. Pluto and Cockatrice boosts your stats based on play time, but unless you are going very slowly through the game, it won't help much more than other defense boosting combos.

Manticore
The Venus combo doubles the amount of hearts you get, the Jupiter combo produces a gas shield that does a very good job of destroying projectiles and weak flying enemies, Apollo and Manticore produces an effect similar to holy water over a larger area, Neptune protects you from being poisoned, and with Pluto you use MP instead of hearts for sub-weapons. Easily one of the best Attribute cards.

Griffin
Not one of best cards, but the Griffin and Diana combo is by far the best of the Diana combos. Also the most costly, so use it sparingly.

Thunderbird
Another not-great card, however the Uranus and Thunderbird summon is one of the most powerful summons in the game, making it exceptionally useful for boss fights.

Unicorn
Insanely hard to get, but it gives you two very useful abilities. Venus and Unicorn lets you trade half your strength for double defense, and Pluto and Unicorn makes you completely invincible (though you cannot damage enemies in this state).

Black Dog
The second most difficult card to get (simply because you have fewer enemies to defeat before you get it, but some very useful abilities. Venus and Black Dog swaps half your defense for twice your strength, Mars and Black Dog gives you a low cost, powerful projectile attack, and Pluto and Black Dog transforms you into a skeleton.

Combinations

 * For more on the card combinations, see the DSS page.