Canterlot Castle, the home of the two royal sisters that literally controlled day and night. It was a literal Palace made of marble and gilded and decorated by the finest artists in all of Equestria. A bastion that represented the might of the country, guarded by the best that Equestria had to offer. Her royal guards, both day and night, were some of the most effective soldiers and law enforcement officers had to show off.
Every entrance was guarded by a pair of royal guards and patrols constantly went throughout the castle for security reasons. It was a natural state of affairs for the castle. Of course, everypony knew and could very obviously see this. Guards weren't the only ponies that would be roaming the castle either. There would be maids, janitors, culinary workers, gardeners and many other various types of support staff for the castle. There were enough ponies inside that one or two extra ponies more than usual would be hard to determine. It was very easy to tell who was who though. Royal guards obviously wore whatever guard uniform their service dictated while the other support staff either wore collars, suits, maid outfits, or whatever else was dictated.
When Snowball trotted up to a wall that bordered and cordoned off the castle to the public. The wall looked like it was made of the same stone material as the rest of the castle, some form of white stone that was polished to a slight shine. He trotted alongside the wall until he found an opening that would let him in closer to the castle. This opening had a guard shack on each side of the entrance and had a singular royal guard standing in each shack.
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The royal guard inside of entrance control point #13-1 was bored. Being an Entry Control Point guard was easily one of the worse possible duties and he was assigned to work it constantly. It was all because he was a stallion. The stallions in the royal guard were heavily outnumbered by the mares in the royal guard (just like everything else in Equestria) and only the stallions were assigned ECP guard duty. Horseapples, why couldn't he get a law enforcement shift for once.
Everypony knew why stallions were given the ECP shifts. They were meant as eye candy for the public. They still had the same training and capabilities as the female royal guards and were capable of performing every other royal guard task and shifts but they would most likely just get light duties like ECP. The few times any stallion got a serious duty shift, nopony would take them seriously, being a stallion.
There wasn't much to working a ECP shifts. It lasted two hours and you had to stand there, looking pretty for the public while stopping anypony from entering. Because of this, it was quite hard to do. If you weren't actively doing something like addressing somepony, you just had to stand there. Which means, you are standing there... For two solid hours.
It was quite easy to just stop paying attention to the world and just sort-of fall asleep standing while still partially keeping a little bit of consciousness. You just had to wake yourself up when you noticed anypony approaching, something that was also pretty easy to do in Canterlot because the roads were made of cobblestone and hooves were very audible with the stereotypical clip-clop noise. Even if you didn't hear somepony trotting up to you, you would certainly have to address them when you noticed them in your line of sight.
Unfortunately for the royal guard at entrance control point #13-1, when Snowball trotted right in front of him, mere meters away, Brass Bushing stayed in his state of half-consciousness/half-unconsciousness state , doing nothing. Snowball even trotted over to look at the other royal guard in the other royal guard shack that was positioned a dozen meters away from the other one and went completely unnoticed. When there was no reaction from the other royal guard, Snowball just trotted right into Canterlot Castle's courtyard, unabated.
Nothing had roused either guard to a state of full consciousness/alertness because Snowball had made nary a noise trotting up to them and when he entered their lines of sight, he blended in perfectly with the background of the snowed in status of Canterlot. Even in a state of full alertness, it would have proven quite the challenge to spot him against all of this snow.