The studio behind the series is Frederator Studios, the same guys behind Adventure Time. According to the studio’s head Fred Seibert, he indicated that the show in based on a world-famous video game of the last 30 years, in which Castlevania fits the bill perfectly as the first game was launched in 1986.
A report from Collider also revealed that Frederator Studios owns the right the Castlevania for at least a decade. This is also coupled with an interview back in 2015 in which producer Adi Shankar revealed that he was working on an animated miniseries based on Castlevania with Frederator Studios.
However this was back in 2015 and now that we’re in 2017, safe to say that things could have changed greatly from then and now. There’s no confirmation yet, but it is certainly quite a compelling argument that the series could be based more on Castlevania than Mario, but what do you guys think?