Types. By 1350 medieval fortification design had reached its apex. Little change would take place for more than a century, until the development of gunpowder artillery began to have an impact. Medieval fortification had three major functions. The most-common one was serving as the fortified residence of a powerful lord, which is the proper meaning of the word castle. Fortifications were used also to defend towns and cities. The third type of fortification, royal forts with small garrisons to defend the frontiers of a realm, was fairly uncommon. There often was overlap between the basic types, such as the well-fortified city of Carcassonne in southern France, which served to block the key invasion route from Spain, or the royal forts built during the English occupation of Wales in the thirteenth century, which became the castles of nobles in the following...