How can i make a telegraph like code book for my world?

In my medieval fantasy(i know very original) there are these towers that in the glory days of the empire stretched the whole country,one each 20-30km.They would function nearly the same as the irl chappe telegraph,so by using long mechancal arms at the top to make signals so that other worker in the tower can relay and by that way speed up comunication.I know its not greatly reliable but its cool and they use it mostly for kingly stuff or emergency(sometimes a rich fellow can send a few codes to his dear wife if the workers are greedy).But i have a problem of how to come up with a sort of hanbook of codes,i had in my mind the idea of going like A1,A2,A3...to maybe A10 and then doing every letter of the alphabet but then still i have to decide what words would those codes mean and i dont know which words i should use or even if thatl be enough and i dont want to start copiing the dictionary if im not 100% sure it will be good.

Any help is greatly appreciated.Even better if anyone has a better idea then using codes like that is welcome to share it.

Thanks in advance

Jani1815.