How do you secure carrier pigeons?
We have people with keen enough an eye and an ear to detect these pigeons. Trained as they are, they sit in pairs in the highest tower of your castle, ever watching for pigeons. Not all pigeons, your majesty, only what we call “egress” pigeons, who fly in a direction clearly meant to depart your kingdom. Targeting only “egress” pigeons is easier because it saves on the drugged arrows they use to fell them.
Then once a pigeon is felled, the trained hounds are released. These specially trained hounds find the pigeon and bring it back into your castle unharmed and intact. Then, the pigeon is take to a special room where it is left to recover the effect of the drug. If the message carries the royal seal, which only your majesty wears, then it is reattached to the pigeon and sent – while a cryptologist reads the rest of the messages and deliver it to your majesty after it has been duly decoded.
After you get to trust our cryptologists, you may order them to perform an action on your behest your majesty, for example, to burn the message so it never reaches its destination, or to send it unharmed, based on its content. Some messages may not be of interest to your majesty, and may be taken to one of your trusted viziers for consultation to await their decision, so your majesty may be free to rule the kingdom. Others may be delivered to your majesty directly, while others may just be copied verbatim and saved for later reference.