Beacons

Key locations across the Sundered lands are equipped with Beacons, a way of swiftly sending messages at night over long distances using light sources.
Each beacon uses a continual flame spell cast upon a crystal at the heart of an array of curved polished metal sheets and focusing lenses, with a metal shroud around the crystal to cut off the light it creates. The resultant flashes of light are visible for enormous distances in clear conditions, and other beacons in line-of-sight can easily read the message encoded in the flashes and re-transmit it onward.
Beacon operators can swivel their mirror arrays to either focus upon a single other beacon, or allow several to see the light they produce. This allows messages to be sent across the network or to single destinations.
By long-standing agreement the beacons are considered independent and sacrosanct, with each located atop a small keep with squads of guards and operators to protect it in case of trouble and able to call upon nearby troops from whichever kingdom it is located within. Should there be an attempt to capture or interfere with a beacon, its operators are usually able to get a message off to nearby beacons and towns in time to summon reinforcements.
The operators of the beacon network consider themselves in the service of the entire wider realm, rather than any single kingdom, and use complex codes and authentication signals to ensure their messages cannot be falsified or interfered with. They send messages of crucial import, such as famine or disaster, as well as more commonplace commercial messages, which rulers and powerful companies are able to pay them to transmit.
All of the beacons are organised together into an arrangement like a guild or corporation, with each beacon receiving payments for messages it transmits and receives, as well as support or subscription payments from the kingdoms and wealthy companies which require swift access to the news of the realm, which can result in great fortunes being gained by individuals and companies able to get the right cargoes to the right ports before competitors.