@Lord Griffin
 
You’re basing your assertion off the scene where Luna flew in through the shield around the city, right? So Luna came in from outside Canterlot. That doesn’t mean Luna doesn’t live in Canterlot. It’s a safe guess that Luna was out searching for the threat against Canterlot like Celestia was with her telescope.
 
I don’t know why you assume that Luna lives in the ruined castle in the Everfree. Why would she want to live in a crumbling dirty ruin with no pony around? Don’t you think that after being separated from her sister for a thousand years, both Celestia and Luna would want to live together where they can interact everyday?
 
Or do you say Luna can’t live in Canterlot because she apparently didn’t notice the changeling invasion? Well, that’s a widely commented on flaw of A Canterlot Wedding. Make up whatever in universe reason you want to to explain why Luna didn’t show up to repel the changelings. Luna was asleep, playing video games, listening to loud music, still searching outside Canterlot, etc. The real reason is that there was not enough time in 44 minutes for the story and to show Luna doing something significant.

Beau, watch A Canterlot Wedding again. Luna VERY SPECIFICALLY does NOT live in Canterlot. We don’t know where she DOES live, but it’s NOT with Celestia. Luna’s home is most likely the Two Sister’s Castle in the Everfree Forest.

To be fair, Luna does live pretty far from Ponyville, and the forest. She lives all the way in Canterlot, or possibly the moon.

I thought the same. ^^  
Would be cool to see Luna kick Trixies plot with some real powerful magic. ^^

^correct. Why is everybody else assuming that Luna lives in the castle. She DOESN’T. The best fan guess is that she lives in the Two Sister’s Castle in the Everfree Forest. Also, she’s a master of MANIPULATION SPELLS. You know, the kind of spells that Trixie was focusing on?

Does Twilight even know where Luna lives? It doesn’t seem to be in Canterlot.

uh, the train station to canterlot is in ponyille guys. INSIDE THE FORCEFIELD

yeah, this is stupid… the thing is Luna isn’t really reliable about magic.  
And Twilight will not be able to contact any on the Canterlot BECAUSE SPIKE IS INSIDE THE FISHBOWL.
 
Zecora is actually next best thing for Twilight at that moment.

someone should add “haters comments” tag

Luna could yell at the barrier.

@Impious  
The latter prolly explains the former.

The last time you tried strange and powerful magic, you got sent to the moon for 1000 years.

I thought this too. And again at the end with the moon I thought there’d be a quick shot of her going across it or something.

Sad feels…

I think she needed a more immediate character to go to, also Zecora needs more screen time

@clowndicks
 
No, she’s an apathetic brat that watched her sister get crushed and her capitol get sacked and did nothing to stop it.
 
Besides, she’s probably sleeping at that hour.

That’s what you get for being so distrustful during the Premiere.

Well, Luna would propably be asleep during the day anyway.

@Impious  
Luna still a miserable husk who can barely function with the general public?

DESMOND THE MOON BEAR

Could she even get to Canterlot?

Sorry Luna, but you’re not important enough. And you wouldn’t help her anyways.