Tobruk is best known for the two sieges during the Second World War, but its military significance starts earlier. The importance of the natural port, which is well-sheltered against the northern winds that are prevalent in the Mediterannean world, was already understood by the Byzantine emperor Justinian (527-565), who built a fort on the site, Antipyrgon. It was part of the Ananeosis, the project to reorganize the Cyrenaica.
Only a small part of a wall can be seen today, and if you go there, I’d suggest you devote more time to the war cemeteries. Nevertheless, I made a small webpage on Antipyrgon, which you can find here.