Dear Shining Armor,
I do appreciate your efforts at guarding this little ongoing diplomatic convention that's sprouted up in your city. You have, to your credit, attempted to do your job as well as you can in the face of uncertainty, impromptu scheduling, and your inevitable paranoia about whether my own goals include replacing your rulers with some kind of terrifying parasitocracy.
But with that said: your ponies just aren't very good at this whole 'guarding' thing. This is a fact, not a taunt or a threat. I had one of my changelings mingle with your guards, just to see what she could get away with. It took three hours for any of them to notice that she was out of place. Three hours, Shining Armor. Do consider the damage I could have done, if I was so inclined—or the damage that could have been done to my changelings. You have some inkling of what we can do and you haven't even set up passwords, let alone real discipline for anything that doesn't require standing in one place for long periods.
You can fix this. I'll be expecting that you do, as one of your future superiors. If you decide that you can't accomplish the task yourself, I can loan you some of my specialists to correct your guards' deficiencies. Isn't that an embarrassing thought? You can consider it extra motivation.
Yours in vague bemusement,
Queen ChrysalisP.S. There's no need to reply. I'll know how things are going.