User blog comment:B214/Question and Answers 2/@comment-24866657-20140605130713/@comment-1359322-20140605131119

They travel through the Dragon Gate, also what do you mean the Dragon Gate didn't happen in Volume 14?

"……Rather, that magic-circle, I have seen it before. Yeah, it’s identical to the thing we used to summon the gigantic Dragon-King, Midgardsormr. Saji mutters. “—The Dragon Gate?” Yeah, it’s the Dragon Gate! The one which calls the powerful Dragons! When I remained in the dimensional gap, I was originally meant to be summoned through this! The Dragon Gate is emitting a green colour. Doesn’t the Dragon Gate give out the colour of the Dragon which is being summoned? I heard that Ddraig is red, Albion is white, Vritra is black, Fafnir is gold, Yu-Long is green, Midgardsormr is grey, Tiamat is blue, and old-man Tannin is purple."