Twilight hovered just a little distance from the dome's surface, observing it. Surprisingly to her, it looked strangely similar to her brother's magic. Purplish, almost pinkish in colour, though not as smooth as the kind of spells Shining used. Instead it looked like it was almost segmented into large and elongated hexagonal pieces, or like a pattern was woven inside it in that shape. It somewhat resembled something an insect might have built, like a hive of some kind.
There was something like a faint buzzing to the spell. Not seen or heard, but felt in the texture of the magic that made up its breadth. It was not static, rather it seemed to be constantly scanning itself. That, among a few other elements, made it feel like quite the advanced piece of work. A complicated construct of arcane engineering, likely powered by some powerful artefact and hopefully not by living unicorns, though one was almost certainly needed to cast it in the first place and perhaps keep it in check and going.
Simply passing through was not a simple thing, it was quite risky in fact. Even if Twilight could do so without the barrier itself recognising anything had happened, and she could, the guards wouldn't miss the rippling on its surface or the empty hole in it she might have generated. Teleporting past it however was out of the question, it would immediately set the spell off if it even worked, and using any kind of magic on the inside of the dome from outside would certainly lead to the same kind of result.
Twilight considered approaching the problem from another angle. She could stack enough spells on herself to make her presence undetectable even by the barrier, but that level of shielding would go beyond simple camouflage. It would require removing herself from reality so much it may have made it significantly hard to come back from it afterwards, especially when she still needed to be stealthy while doing so.
She had underestimated how hard it would be to get inside the Empire, she had to recognise. The files she'd received did mention some form of perimeter defence, but she'd assumed it would be more akin to a wall or a patrol of guards. She'd been surprised upon first seeing the dome, though she'd kept herself in check. She hadn't expected the city to have more advanced security measures at its border than Nightmare Moon's castle itself did. Maybe it said something about the ponies in charge of the two.
The guards beyond the barrier seemed unwavering in their dedication to their task. Most likely, they had no choice in the matter. Twilight carefully probed a little forward with her magic, trying to get a feel for the dome without disturbing it enough to set off motion of any kind. She could almost make out the matrix of interlaced enchantments interwoven within the barrier's structure, but pushing any further would have triggered it all. It was like trying to push through several separate layers of spider webs without disturbing any of them. At some point she just got to a dead end where every further movement would touch something regardless of the direction.
Twilight pulled back her magic, and pulled herself a little further away from the barrier too. It was a little unnerving to see the guards stare through her like they did as they scanned the area, but she was sure they couldn't actually see her. They would have taken action against her already otherwise.
What she really needed was a distraction, she'd realised that pretty quickly. She could pass through the barrier without alerting the guards at all, but the barrier itself would feel her and someone soon would know. Or she could completely stop the barrier from noticing her passage, at the cost of it being visible to anyone watching. So what she needed was for the guards to be looking elsewhere when she passed through.
Waiting for a guard shift that might never come was entirely unreasonable, and so was immediately trying to erase their memories after she was inside. It might have been viable if there was just one, but with that many she simply wouldn't have been able to do it in time. On top of that, whatever magic was controlling them could probably make it much harder for her to mess with them mentally. She'd never studied Sombra's spells too well, there was a tragic lack of resources relative to that topic in her Equestria. Maybe she'd interrogate him if he ever came back.
Twilight shook that thought out of her head, and felt her mane sliding over her neck. It had gotten a little longer, she'd noticed, and she'd started getting a little taller again too. She didn't like the idea of turning into Celestia, not in terms of actions and thoughts at least, but if it happening on the physical side was inevitable she at least hoped she'd be taller than the alicorn by the end of it. Looking down at her instead of up didn't sound so bad. She'd already gotten used to it, all things considered.
Again, Twilight had to stop her mental meanderings. Creating a distraction wasn't hard, and there were many ways she could go about it. But a simple illusion spell would leave the guards questioning who'd cast it afterwards, and the same went for simply throwing something on the ground or moving some snow. Regardless of what she chose to do however, the most important thing would still be acting quickly. The distraction would only hold the guards' attention for a very short while, maybe less than a second.
It was enough, if she kept her focus and did things properly. Her horn shimmered in anticipation, as she prepared both sets of spells. The distraction would be an illusion in the shape of a changeling. Hidden at first, it would move some snow with the beat of its wings, accidentally step into the snow shortly after, and disappeared back into the blizzard as it ran away. Once she was sure she could get inside the dome fast enough, she set things in motion.
The guards all focused on the very small puff of powdered snow that rose from the ground a third of the way from the blizzard's edge to the barrier. Twilight waited. The step came. As bolts of magic surged from the pikes the guards carried and the horns of those who had one, a small section of the barrier thinned and greyed for a fraction of a second. When the commotion was over, Twilight was already inside. She looked at the guards, but none seemed to have noticed anything. Reassured she turned around, and began to fly above the Empire.