A Summary view
Since the beginning of time, human beings have been devising ways to defend their family, their community, their assets, and their lands from other enemies. This is how the first fortifications arose all over the world. The fortifications, the walled Greek and Roman cities, the medieval castles, the star-shaped forts... However, these fortifications are not the ones that occupy this article. These were used to defend specific geographical sites, often in high and strategic sites. Fortified lines are a different type of defenses: they are not punctual, they are linear. Thus, in antiquity we can highlight Hadrian's wall in Roman Britain or the Chinese wall.
The aim of the lines was somewhat different than that of the point fortifications as well. The lines were intended to clearly mark a border between two very different worlds (Romans-Picts, Mongols-Chinese...), furthermore, only very strong states such as Rome or China could afford to create these border-walls. However, these old lines did not have the success that was expected of them. Why was that? Simply because even though they were very powerful states, they did not have enough resources to garrison and supply the lines in all their length all the time. It is for this reason that punctual fortifications were much more used and were much more successful.