It is the 10th Century and the hero, Ibn Muadhe has discovered trigonometry about six centuries before Cartes, and has received a wonderful gift.
A very spiritual Avicenna had rushed by camel and horse from Persia to bring two special stones each the size of a person's hand, which can communicate with each other much like present-day telephones, with exceptions. They can only communicate by a straight line and only through atmosphere. They can work in rain and fog, not that I imagine that's a huge issue in Al-Andalus.
The Caliph of Cordova is fascinated and has demanded Ibn Muadhe establish a communication system as far as possible for strategic purposes.
The problem lies in the fact that this is 10th Century Islamic tech in what is now southern Spain, and, as well, Ibn Muadhe knows the Earth is of course spherical and not flat.
Using this tech and the resources of the Caliph's coffers, how can he get these two magical stones as far a distance as possible? And how far would that be?
I'm thinking tall structures with the stones atop, but hoping for a creative solution using the properties that are allowed and the technology level involved. I'm willing to bend the rules of the properties of the stones just a bit if it can get me a really great distance in a clever way.