Fortification (Civ3)/Fall of Rome/Civilopedia

As siege weapons are improved, so must the fortifications defending against them. Simply put, a change in one
forces an evolvement in the other. The walls around a city must become thicker and taller. Battlements are
surrounded by moats, sometimes filled with water, sharp projections, dangerous creatures -- or a combination of
the three! Even the towers interspersed along a fortified wall must adapt as the weapons of siege warfare evolve;
roofs are added, and firing ports are modified to better serve (or defend against) new weapons.