Twilight Age

What if humanity never fully emerged from the dark ages because of a new light metal resistant to bullets? What if we continued to advance in technology, but stayed in walled cities, kept armored soldiers, advanced the technology of our swords, spears, axes and such? Welcome, to the twilight age.

Timeline
1600: People in quarries across Europe discover a strange new metal. With some testing it is found to be many times lighter and stronger than steel.

1601: Quarries were the metal is found are yielding massive quantities of the metal. Metal is easy to forge.

1602: Metal is named quarry steel.

1603: Armored troops are more resistant to bullets. Weapons can be made sharper and larger.

1604: If made thick enough, can survive relentless cannon fire.

1605: Walls on fortresses covered with quarry steel are resistant to most cannon fire. In England, most forts have walls covered. Construction begins on walls around cities.