Obviously we know that Vampire Killer (MSX), SC4, and X68000 are remakes of the original Castlevania. Haunted Castle is added to this list officially, but I like to think different. Let's recap Simon's escapades:
Castlevania: Simon kills Dracula
Simon's Quest: Simon resurrects and kills Dracula's ghosts. No worries!... unless you go by the good ending where Dracula's hand comes up from his grave after Simon leaves.
In HC, you play as Simon again, but this time, Dracula kidnaps his newly wed wife after they walk off from their wedding, forcing Simon to hunt down and kill Dracula in order to save his love. It makes sense: After Simon's Quest, Simon has settled down, thinking he's finally done with his duty, eventually finding a woman he loves, who he marrys. Dracula kidnaps her to get to Simon. But beyond that, you can look at Belmont's Revenge.
Castlevania: The Adventure: Ends with Christopher killing Dracula. After watching the castle crumble, Christopher walks away. Dracula then flies out of the castle's ruins in bat form, the story ending right there.
Belmont's Revenge: 15 years later, Dracula has built some of his power, and Christopher's son, Soleiyu, has a festival party thrown for him, since he is ready to take on the duty of his father. A deep fog engulfs Soleiyu, with 4 castles appearing off in the distance. Christopher learns this is Dracula's doing, and sets out to stop Dracula once again. When Dracula's Castle appears and Christopher makes his way to the keep, Soleiyu appears, brainwashed by Dracula. Chirstopher engages in battle with his son and defeats him. After killing Dracula, Soleiyu and Chirsopher watch the castle crumble, the battle over.
Dracula took a while to build his strength back, about 15 years. He then kidnaps Soleiyu and makes him his slave to kill Christopher. He then planned to kill Soleiyu as a sacrifice for his power, as Soleiyu admits upon defeat. Assuming Haunted Castle is a sequel to Simon's Quest, it could be the same situation: Dracula waiting a while in hiding for his power to build up, and striking when the time is ready, only this time it's the Belmont's wife.
I doubt the developers of HC even thought of Simon's Quest when making HC, especially since HC's release was a few months after Simon's Quest in Japan. They were just tasked with making an arcade conversion of Castlevania. This theory could be an unofficial way of looking at the timeline, and since Simon's Quest's good ending never received a sequel, we could say we've had it all along... Still waiting for that ending to be followed up on
