dark and light

[Fenral]

In FF3 we find out that the crystals will get dark warriors if light starts to take over to much. .

The problems that arise when light is out of balance were teased during the Syrcus Tower quest, actually, as what began the downfall of Allag. Might've been an accident, or a case of the lore team not communicating with the event team, since the rest of the CT story went right back to the "light is absolute good" that FF3 tried so hard to subvert.

[Felis]

The problems that arise when light is out of balance were teased during the Syrcus Tower quest, actually, as what began the downfall of Allag. Might've been an accident, or a case of the lore team not communicating with the event team, since the rest of the CT story went right back to the "light is absolute good" that FF3 tried so hard to subvert.

There was no "dark and light" balance thing in the CT story.
Xande was pushed into a corner by the rebels and he activated the Crystal Tower. But the energy sended by Bahamut to the Crystal Tower was too much, resulted into a worldwide earthquake. The 4th umbral era began.

[Fenral]

There was no "dark and light" balance thing in the CT story

Not explicitly, but you missed Doga and Unei's explanation for why the Allagans resurrected Xande in the first place. They built their Crystal Tower to provide all the energy they ever needed, and it did. It did its job so well they no longer had goals to strive for, leading to the slow but steady decline of their civilization. Xande, our metaphorical "darkness," was revived to counter out the "light" that had stifled their growth as a civilization.

It's also meant as a reference to the decadence and decline of the Roman Empire, but it seems rather odd that they went out of their way to specifically point out the Tower as a root cause of the problem in a story that ultimately wanted to pass it off as a "light" that should be given to the people of the future. Just my two cents, but I have done the legwork on this one.