The only Dimensions that Cassius hadn't ascended yet were Nightingale, the Heavenly Kingdom, and, obviously, the Holy Sanctuary of Yggdrasil. The first was simply because he was saving it for later, and the second because Angels were very mysterious—their Ascension Tower even more so.
Simply put, they were a risky beginner choice.
Then there was Yggdrasil.
Well… that didn't require any further elaboration, did it?
Yggdrasil was certainly overpopulated with end-game bosses. And as much as Cassius wanted to search for his mother and understand why the Dissonants truly wanted her—and him—he couldn't afford to risk things at this juncture.
As for why he made it his goal to ascend the Towers…
*Ding!*
[Congratulations! You have fulfilled the requirements for the 5th… or was it the 6th Main Quest? Well, no matter, you did it, and that's all that matters!]
{[Main Quest #5]}
