Now that you "prossess" all five parts of Dracula, as well as the Magic Cross, you can enter Dracula's castle.
Go to Donia
You should now go to the town of Donia. To get there, go to the right from Laruba Mansion, using Laurels to help you cross the poison swamp. After you get out of the swamp, you'll be in the Devious Woods. Go to the right and you'll be back in Uta Path, where you got your whip upgraded. Go from the bottom-left to the bottom-right of Uta Path. Go right from there and you'll reach the area under Uta Lake. Go upward and then go to the right. You'll walk past Bodley Mansion. No need to go back in.
Go to the right from Bodley Mansion and you'll be in the Wicked Ditch. The Venus Fly Trap can spit fireballs, so use Dracula's Rib to deflect them before you try to jump up and kill the evil plant. Go to the right and some Harpies will swoop down. If they land on the ground, they will walk toward you while spitting fireballs. Keep going to the right and you will reach Donia.
The only thing that you can buy here is Laurels. There is a church here if you need to heal up.
Go to Yomi
Now go to the right from Donia. You'll be in the North Bridge, where some Fishmen will pop out and spit really fast fireballs at you. Keep going to the right and you'll be in the Dora Woods. Go to the right, past the poison swamp, and eventually you will find some stairs. You have to go down those stairs, or you will reach a dead end. After you go down the stairs and go to the right, you will reach the town of Yomi.
There is nothing to do in the town, and not even a church where you can heal up, so just go to the right to leave town.
You'll be in Vlad Graveyard. Go to the right, fighting Mummies, Vultures, and Phantoms. The Vultures will hover in the air for a moment, then will swoop quickly toward you, so be prepared to jump. Keep going to the right and you will reach the West Bridge, which is the final barrier between you and Dracula's castle.
If you have all five parts of Dracula, and the Magic Cross, you can use Holy Water or your whip and Dracula's Nail to destroy the blocks that prevent you from jumping up to the bridge. Do so, and go to the right. You will soon enter Dracula's Castle, known as Castlevania.
Dracula's Castle is empty, so just follow the straightforward path until you reach a dead end. Use the Holy Water to destroy the blocks that are in your way. Go down some stairs, then keep going downward until you find two sets of stairs. Go up the shorter stairs, then jump to the left to go over the wall down below. From there, simply follow the straightforward path to Dracula's chamber.
When you arrive in the chamber, Simon automatically tosses the five parts of Dracula onto the altar. They burst into flame, then Dracula's body reanimates. When Dracula's body appears, you can immediately start attacking him. The Golden Dagger works particularly well, and you can defeat Dracula easily before he even has a chance to move.
Congratulations! You won the game!