Dracula's Riddles dealt with a number of things:
-Bits of information about what to do in certain situations.
-The locations of certain relics
-Controlling situations in order to get past certain illusions
Veros (in a room) - "Clear a path at Berkeley Mansion with a white crystal." - Means "use this item at this location in order to proceed forward".
Berkeley Mansion - "A symbol of evil will appear when you strike the stake." - Means that the few times in which you use the Stake item, it's in order to get something particularly evil (i.e. Dracula's Remains).
Berkeley Mansion - "Destroy the curse and you'll rule Brahm's Mansion." - Some explanation below.
**The curse creates an illusion and a poison in the countryside.
**The illusion is in the river, because it masks away Brahm's Mansion. Hence "you'll rule Brahm's Mansion", as you will be able to get past the illusion and see it (the Ferryman sees it too, but he's weiiird).
**More on 'the curse' later, as there are more clues.
Berkeley - "A flame flickers inside the ring of fire." - This could mean one of two things, and I'm not even sure on my interpretations, but it could mean:
**There is 'something alive' or 'something evil' inside the magical spheres in every mansion. It's a clue to strike them to get Drac's Remains. (this is my best guess)
**There is a flame power for your whip concealed inside an alcove room somewhere (refers to the sorcerer who powers up your whip with flame power)
**There is a flame item concealed inside an enclosed chamber somewhere (Dabi's Path has a flame item)
Aljiba Woods - "To replenish earth, kneel by the lake with the blue crystal." - Exactly what it says. However, 'replenish earth' means' really means to either drain the lake or remove a curse and see what's below.
Town of Aljiba - "An old gypsy holds a diamond in front of Deborah Cliff." - Exactly what it says. There is a person who gives you a Diamond/ReboundStone in front of Deborah Cliff. What the clue doesn't tell you is that there are two parts to Deborah Cliff; one by Brahm's Mansion (the one with the diamond) and the one further west on the main path, which is where you proceed forward (another clue is related to this other location)
Rover Mansion - "Destroy the curse with Dracula's heart." - You use Dracula's Heart to get the Ferryman to get you to the cursed part of the Dead River, breaking the illusion and taking you to Brahm's Mansion.
Rover Mansion - "Garlic in the graveyard summons a stranger." - The first true graveyard you encounter is east of Aljiba. Should you visit Aljiba's graveyard after getting this clue, and dropping a Garlic into the ground, you will summon a stranger who gives you the Silver Knife. Similarly, on the later graveyard after Alba, you can summon a stranger in the same way, by dropping a garlic at the screen at the end of the graveyard. This stranger gives you the silk bag, thus increasing your laurel and garlic capacity (not sure about the garlic). NOTE: A villager misleads you by telling you "Get a silk bag from the graveyard duck to live longer." Maybe he meant "Get a silk bag from the graveyard stranger. Crouch to avoid the slimes, dragon fireballs, etc. to save your life"?
Brahm's Mansion - "Place the laurels in a silk bag to bring them to life." - Receiving the silk bag item increases your laurel capacity.
Deborah Cliff (not the one with the Diamond person) - "Dracula's nail may solve the evil mystery." - If you do not pick up Dracula's Nail, a set of blocks prevents Simon from crossing the bridge to Castlevania. Having the nail removes a crucial block you need to climb the bridge.
Bodley Mansion - "The curse has killed the laurel tree." - Laurels are increasingly scarce, and salesmen will not sell you more than the 4 you can carry. The reason for this is that the laurels carry some magical properties that Dracula does not want you to have, so his 'curse' has destroyed the means of procuring this item (as well as poisoning the land, driving some villagers mad, and causing illusions in the countryside).
Debious Woods - "Dracula's evil knife blurs Camilla's vision." - You receive a Golden Knife when you defeat the Grim Reaper in Brahm's Mansion. This knife can be used to stun and land a few hits on the boss of Laruba Mansion, Carmilla's Mask. By attacking Carmilla with the Golden Knife, you get a cheap dirty victory without risk of counterattacks, for the cost of a few hearts.