@DoomPony  
I just checked the episode, Shining Armor says: “Cadance has been able to use her magic to spread Love and Light. That seems to be what is protecting it.”  
So I’m theorizing that Cadance is using her magic to spread the feelings of Love. (Like we saw her do to the two ponies in the Wedding flashback) And that is somehow producing the force field. Perhaps it’s providing enough of a boost to the empire’s defenses that it is able to create the shield.
 
Shining Armor’s Love for Cadance and her Love for him could be increasing that power. Like the finale to the Wedding.
 
So in a way, it could be that Shining and Cadance’s Love for each other, is providing enough of a boost to her magical power of spreading that Love throughout the Empire, thus powering its defenses.
 
There. Now all three theories work together with no issues… Right?

@PonyPon  
I thought Cadance was just making a love bubble.

@dynamicv  
Well dang, now I have no idea.

@Poptard
 
@PonyPon  
Hold on, I thought it was the empire itself making the shield. Cadance was just a battery.

@Poptard  
Yes.
 
Honest mistake, I’m sure.

Hey wait, wasn’t it solely Cadance putting up that barrier?

“Raise this barn, raise this barn, 1, 2, 3, 4. Together we will–who in tarnation put a force field around mah barn?!”