Forget radio, what if the Romans had thought of Semaphore? In the time of
Napoleon, semaphore towers could convey official messages from Paris to
Rome in 20 minutes. I imagine that a just-sent "The Huns are approching.
We are burning all records," would bring help faster than a guy riding a
horse as fast as possible. An elaborate system could even have had room
for a newswire type of service, where space was available, with slaves
copying down messages to be read to the masses.