Jan
7th
2013
The pony capital? · 9:49pm Jan 7th, 2013
I just realised something.
In most fics that deal with Equestrian history (or at least most fics I came across) it is assumed, that the Castle of the Royal Pony sisters was the former capital and a place were the court resided. Then Luna went NMM, and the batle ruined once glorious city, so Celly moved her seat of power to a new place: Canterlot.
But if that's so, than what about the Traditional Royal Canterlot Voice? If Canterlot wasn't the capital back then, than why would they attribute it to royalty? And if it was, than what really was the Castle of the RPS? A summer villa? Maybe something like Versailles, while Canterlot was more like Paris? Either way, my hedcannon is ruined now...
Methinks you're over-analyzing this. Which actually shows your devotion to the show which, sadly, cannot be said about some of the recent episode writers.
Either way, you're partially right. In that way, that since Canterlot Voice got named just so, than it means that the city had to be the capital of Equestria long before NMM.
However:
I have to agree that I've first heard this idea voiced by you just now. I have probably missed the fics you are talking about.
As for my own, I have never thought about it in that much detail. 
One things certain: something dreadful must have happened in the old castle for it to be ruined so and swallowed up by the Everfree Forest.
Well I have sort of three original capitals, with some sort of meeting place between them. Everfree was the earth pony castle, Cloudsdale the Pegasus one, and Canterlot the same for the unicorns. All were near each other and they were cooperating, not living with each other in harmony quite yet. Then Discord came along and things got complicated. After the sisters won they ruled From Everfree since everypony could reach it easily. Then Luna went crazy and the Everfree forest became the super dangerous place we know. Thus the royal Canterlot voice actually goes back to Platinum and her descendants, so there isn't really a problem.
687730
Maybe I am over-analyzing this, but I guess I find it fun :] Not sure if it's a sign of devotion, or of simple nitpickiness (is there a word like that? :P)
I'm almost certain Within and Without written by Cloudy skies is depicting the castle in that way (or I remembered it weirdly). And as far as other fics go, to point them out I'd probably need to go and reread most of the fics I've read that dealt with alicorns or Twi (since history was usually mentioned in fics like that).
688169
Hmmmm. So you think that the 3 tribes actually build separate cities, right after learning that they need to work together to survive? That seems kinda counter-productive (though I guess some animosities still might have remained...).
But apart from that - so you're saying the Royal Voice isn't something from the Royal Sisters era, but rather even earlier relic, from the 3 tribes era? Haven't thought of that. That actually does sound like something them silly unicorns would devise...