The Rise and Fall of the Visual Telegraph (2017)

For the discworld alternative, check the clacks [1].

[1] https://wiki.lspace.org/Clacks

It’s also fascinating this didn’t existed 300 years sooner (with more towers because of poorer telescopes). Communication of information across a kingdom is so important

Ah, so that's the origin of semáforo in Spanish, which the current meaning is "traffic light". And a semaphore on programming, too.