Beacons were used to signal long distances before the invention of the telegraph and telephone. They were often placed on hills with good views. In daytime the smoke of a fire would be used to make signal, and at night the light of the fire might be seen. However, it would only be possible to sound the general alarm. They were often organised into complex networks to warn of invasion. They may have been temporary or have had a standing garrison if in continuous use. Since the views did not vary too much through time prehistoric sites might have been re-used as a Medieval or Post-Medieval beacon site, e.g. the Iron Age Ros Castle hillfort owned by National Trust.