They have mentioned on a few occasions that they were interested in eventually implementing Eureka- A subterranean treasure trove/town/necropolis that existed in the basement of the crystal tower people may remember from FF3jp.
If you go back to the cutscenes of CT, Nero is there for a singular reason, to locate a specific signature of allagan origins. Using his special compass.
At the end of WoD, he ultimately gives up on such a folly of a quest, as he never got the compass to "ping" what he was looking for. He discards it off the ledge into the abyss surrounding CT.
...and as the screen fades black, you as the observer of the cutscene can barely make out a steady pinging noise... The compass was finally detecting its target nearby, deep below CT.
Also, a fanmade artwork was made as a suggestion, that the tower was fully crystal shaped, light side above ground, dark below. They certainly could have taken that to heart in their plans.
I couldn't find the original image but I tossed this example together in paint. Sorry it looks rushed, but you get the gist.
Not tough to figure out what to expect, lol.