Great concept, great execution. Celestia has been watching Twilight Sparkle since her birth because she already knew her. My thought is that modern Celestia has been communicating what is happening in the future back to her old self via Starswirl the Bearded's time travel spells. That is how Celestia knew a thousand years were needed for Nightmare Moon to return to Equestria. But this works very well also.
And I tend to agree, Celestia and Luna started out as unicorns and became alicorns after being linked with the Elements of Harmony. The two sisters got their cutie marks back in the age when all unicorns were needed to raise the sun and the moon.
I found this to be really awesome because of the unique idea and the overall style and design! I love the way that you made the guards in a negative sense and used the princesses as fillies! The way you created Twilight is very well done, making her look very brave and majestic! The character design is also very nice and your style adds alot of depth to this piece! This is pretty awesome and I hope to see more! Great work! Kind regards, Princess Luna