(Geffen came first before this card btw, and Ragnarok Online is huge in Japan, so it's not surprising if this card is meant to be a reference to Geffen)
Geffen is a city of mages in Ragnarok Online, and the layout of Magical Citadel of Endymion is practically identical to Geffen, down to having the same coloration.
Also, not sure if this was a coincidence or not, but the Mage class' highest form in Ragnarok Online, a Warlock, has clothing style that resembles Endymion.
Probably intended, as some sort of "thanks for the love" reply to what Yugioh did.
Anoymous
Nathan Philip Dunn