This tutorial talks about the classical era
The classical era is when your technolies bring you to a more advanced level of sience
But you should also start thinking about what sort of units you should build because most wars start at this era
For technolgy you start researching mathematics why
well it lets you build a world wonder and you can start building catupults (you need iron working for catupult )
building now you should start to build your first wonders you could or biuld the pyramids or stonehege
EXTRA When you get iron working and a sorce of iron is within your borders build a mine and then immidetly start building a catupult and a swordmen Now you should also think about ecomimic reservs and social policis .