The Zenithia trilogy (Castle in the Sky trilogy) is the general term used to describe the 4th, 5th, and 6th installments of the Dragon Quest series; arising from the presence of a Zenithian Sky Castle found in each game. Despite this, the plots of these three games do not specifically reference the others in the series. This detail separates it from the Roto trilogy.
Another common feature of these three games is the dark underworld known as Nadiria in the DS remakes. This evil counterpart to Zenithia is the realm of the main final boss of each game (not including bonus dungeons).