I found a nice history of warning signals from the US Lighthouse Society. The problem seems to be that the things, such as fog, that interfere with light also interfere with sound propagation. One of the possibilities that you should consider is that there will exist times with communication blackouts. In such times, a pony express type service for critical signals. They also developed underwater sound signaling from shore to ship to shore using coded signals. The method was in use until WWII because everyone can hear the sound.
There are multiple problems with sound. First, its distance is weather specific. Signals rated for four miles often only made it one or two in the wrong wind and climate conditions. The same sound signal could be heard eight miles away in other conditions. The other problem is that sound can ricochet in adverse weather. It was a frequent problem that sea captains couldn't determine the direction that the sound was coming from. In fact, the bending of sound was taken advantage of in several places by constructing objects to trigger purposeful bouncing.
Finally, the transmission is going to be slow because you will have variable ranges where towers will have to be silent. If multiple towers are transmitting it may be impossible to distinguish signal from noise.
Even canons had a range of two to six miles depending on the weather.
For signal propagation by water, it was discovered that coded messages could be transmitted up to fifty miles by underwater signals, originally they were bells.
The actual method of signal communication in use in Europe was a variety of forms of semaphore and horse with underwater signaling for ships. The U.S. Pony Express could cross the United States in ten days. You do not need that. You only need that for critical signals in adverse weather. If you had to cross water, you could use submarine bells.
There is a massive advantage for semaphore over lighthouses. See the BBC article on it. It is a bit less robust than a lighthouse but it has many advantages over a lighthouse. You can restrict the line of sight for military purposes. The Napoleonic system has 98 combinations of possible signals using a trinary system. That made it faster than Morse. However, you should read Terry Pratchett's Clack's system. You should read The Fifth Elephant where it is introduced or Going Postal where it could be thought of as a primary character or a McGuffin.
If it were me, I would ask myself a set of questions.
- What signals could be delayed when semaphore was down?
- Is the added cost and slower speed of a lighthouse worth enough to not use the faster semaphore system?
- Is there enough signal volume to partially include a Pratchett like Clacks system using light?
- Who is paying the bills for a sound or pony express system to cover briefly befogged or blizzard conditions?
- How often would the alternate systems be down such as during a blizzard? There may be historical data on this in Europe.
- A number of telegraph confidence games were played on people by compromising the telegraphy system for profit during the 19th century, even before electrical telegraphy. The first steganography issues were semaphoric. The first "viruses" were semaphoric not computer networked. How will you secure your system from nation endangering fraud?
- Is the system robust to war? The internet was designed to be robust in the event of nuclear war. It is quite a bit more fragile now for many purposes but for standard purposes, it is surprisingly resilient. Can I bring the entire network down by capturing one tower? Does the system collapse with the sudden heart attack of a single semaphore operator?
- In the much slower world that used to exist, what is so critical that an hour of fog would change the world?
I would remind you that during the Cuban missile crisis there was no direct communication system between the United States and Russia. They passed handwritten notes between the President of the United States and the General Secretary of Russia. That was during the twentieth century. Napoleon's system was actually faster than the US-Russian system. That is why the hotline was built. To understand how slow the world used to be when Kennedy was assassinated there was no way to get the news onto a visual broadcast at the time of the assassination.
It used to take twenty minutes to half an hour for a camera to power up for transmission. Instead, the transmitted a blank screen with sound only. It was essentially a placard being transmitted with sound. The news networks were not notified by phone either. They received wire signals, which is still telegraphy.
The news in 1963 used a series of bells to notify them that a news story was being transmitted and its priority. When Kennedy was shot one reporter happened to be in a car with a radio phone. Otherwise, it would have taken at least ten more minutes to relay the message. It still took ten minutes to go from reporter to a signal being received at network stations.
Newspeople also blocked transmission. When the first reporter used a payphone for the one private line out of the hospital he had the other side not hang up. Under the old phone system, neither party could use either phone until both parties had hung up. By keeping the other side off the hook, the one private line out of the hospital was disabled. No other reporter could send their stories at the hospital, they had to run to other locations to find a phone.
A canon or sound-based system could have that problem. You can block the signal by transmitting sound with no purpose except to block the relay stations surrounding the transmitting station. You could send the message "The President has been shot. In the beginning, God created the Heavens and the Earth," and continue through the Book of Revelations. Terry Pratchett took advantage of that problem in Going Postal.
Look at the problems at Pearl Harbor or on 9/11.
Sunlight, with telescopes, is your friend. Relatively frequent lighthouses would be a little less friendly. Canons and horns that blast over a wide but variable radius will work but be costly. Horses and submarine signaling systems are reasonably stable and reliable.
You can build a mixed system for redundancy.
EDIT
With regard as to how semaphore is better, consider two optimization functions G and U. Let us imagine that the purpose of G is to train an Olympic sprinter while U is to train a high school basketball player. They will share many elements in common and differ on a few key elements.
When engineering something, the goal is almost never to build the best thing but a thing fitted to the service it is being used for. A simple example of this was the PC wars of the 80s.
By any technical measure, the computers by Commodore and those by Apple were vastly superior to those produced by IBM. Commodore went bankrupt. Apple would have gone bankrupt but Microsoft injected massive amounts of money into Apple, acquiring 40% of the company. It did so because Microsoft would have had a monopoly in the operating systems market and its shenanigans would have instantly become illegal under federal law. Fit to purpose is different from building the best.
Terry Pratchett's Clacks system is a McGuffin or maybe even a passive character. It is, in practice, a lighted semaphore system. A traditional signal light using something like Morse code would be comparably slow. However, it begs the question of whether something like the Clacks would be built. Pratchett uses it as a metaphor for cell phones and to explore the information society we have been building. It never serves an actual engineering purpose.
In a world without lightbulbs, everything stops at sundown. There is no need for night signaling. The only organization that could possibly need signals at night would be the military. If a light suddenly came on at midnight, every intelligence agent in that country would know that it was a critical military signal and it would be recorded.
Without electricity, the world slows down. While it may be valuable for merchants to have price and volume data on a slow delay, not much else requires near real-time communication in such a world.
Directed light is more fragile than semaphore and costs more. You need fuel. It is costly to repair. The towers cannot be made of a flammable material so you have to have higher building and repair costs. Your operating crew would be your engineering crew so you would carry higher levels of training and salary costs. An actual Clacks system, such as Pratchett's would be very costly because of all the mechanical parts. Because they could not be engineered at the site, you would also have to carry a significant parts inventory. Your people would be insanely skilled. Essentially, you would be scooping up the nation's watchmakers.
Smoke signals and light signals have been in existence for thousands of years, yet semaphore replaced those systems. I believe the only remaining smoke signal in use is with regards to the election of the Pope and that is a binary communication. Light signaling with lasers exists but only where point-to-point, high criticality messages are involved.
If fidelity and distance are your concern, then a light signaling system may be better. However, the bulk of the messages will be "the price of iron in Kent is three pounds per ton of ore," or "send money fast." What about those messages require high fidelity?
What purpose is the builder of your system trying to optimize?
For Pratchett, it was a discourse about modern life.
For the real ones built before electrification, they were all semaphore systems. The most famous is Chappe's because it only took two hours for a signal from Venice to reach Paris, but Hooke, Edgeworth, and others built semaphore systems throughout Europe, Canada, India, and the United States.
Lighthouses exist to send one narrow, repeating signal. It is "if you lose track of the position of this very bright light, then you will die." It is a high criticality message. Semaphore would not work for this as you could not see it at night when it is needed. Bells and cannons were used for this on foggy days but they did not carry other messages. If they had then they may have confused shipping since bells are distinctive.
Lighthouses are preposterously expensive, especially when compared with a wooden tower, a telescope and a few mechanical arms. The fact that light, smoke and sound signaling was replaced by semaphore in the real world over dozens of countries, independent of each other, tells you they are inferior.
