Without any inclination of who is responsible for developing Fortress -- if anyone is developing Fortress -- it's impossible for us to test the credibility of this video. The original poster, YouTube user DiipuSurotu, doesn't mention where the video came from, only stating that it's a tech demo for an "official, but currently unannounced sequel to Final Fantasy XII." We've contacted Square Enix to see what they think of the footage.
It doesn't really look like your traditional Squeenix fare, but the character models certainly resemble those featured in the leaked concept art that surfaced last year. Oh, also? There's totally a Chocobo at the 1:15 mark. That's Final Fantasy-y enough for us.
[Thanks to everyone who sent this in!]