Castlevania & History Special - Did You Know Gaming? Feat. Markiplier
According to The History of Castlevania: Book of the Crescent Moon, the Belmont family was originally planned to be called the Dante family, with Simon Belmont originally being "Peter Dante, Vampire Killer and grandson of Christopher Dante".
Contributed by MehDeletingLater
In an interview with series producer Koji Igarashi, found within the Castlevania: Aria of Sorrow's official Japanese strategy guide, he was asked about the character's name Soma Cruz. He said that he wanted a name which would translate well for international players, and that "Cruz" was chosen as Penelope Cruz had just visited Japan, while the name also had associations with the word "Cross". Soma was taken from the name of the Indian ritual drink "soma", while it is also the name of director Murakami's son.

Aria of Sorrow direct Junishi Murakami said this was just a coincidence however, as the kanji of both names differs. When he first saw the name, he disagreed with the idea, but Igarashi simply couldn't think of anything else.
Contributed by ProtoSnake
There is a Yu-Gi-Oh! card called Vampire Hunter which is likely based off the Belmont family.
The Castlevania castle can be seen in the art's background and the card's name translated from Japanese is Vampire Killer, the name of a whip passed down through the Belmont family.
Contributed by Psychospacecow
The Entrance Hall area has almost exactly the same layout in most of the games in the series.
Contributed by ummwat
In Japan, the Belmonts are known as the Belmondos. Also, Trevor Belmont's name in Japan is Ralph C. Belmondo.
Contributed by peanutgamer
In the High Valryian language, from Game of Thrones, the word "Chain" is "Belmon". This is meant to be a reference to Simon Belmont from Castlevania, since he can get a chain whip.
Contributed by SRJ