Meracydia/Miqo'te/Azys La speculations (spoilers)

[Fenral]

You're forgetting that Syrcus Tower was actually quite recent, one of if not the last thing the Allagans built before the second reign of Xande. The difference in architecture between the Tower and Dalamud is less a difference of advancement, and more a matter of aesthetics, as the former was meant to be visited by the public, while the latter is a military facility with little use for fancy trimmings.

While the dimmer lighting makes it harder to make out, if you take time to compare the architecture of the Fractal Continuum to the architecture in Syrcus Tower, there are actually far more similarities than you'd think. Enough that they could easily be contemporary, even though the Contunuum feels much newer due to it's location in Azys Lla.