Military innovation is usually a series of measures and counter-measures. You invent good swords - I invent more powerful armor. You invent walls - I invent catapults. You invent Castles - I invent explosives to bring them down. And so, you will be forced to elevate your city defenses to a whole new level yet again. The development of new siege tactics leads in turn to the development of the most powerful defensive structures of the old world, known as Renaissance Walls. Also, a new field defense system, the Fort, is invented to provide strongholds outside of cities.