I'd like to stay in the classical era because I want to fight a classic war, without guns and stuff. Is is possible to remain in the classical era and if it is possible, how do I?
There are several available mods for this. They essentially delete the tech tree beyond a given era.
Playing at the slower speeds--Epic and Marathon--gives you more turns during each era. This allows you to better utilize a tech advantage and wage war since you have more time to move your units and take cities before your enemy learns new techs.
It won't halt progress as the mods do, but it does give you a partial effect (and a slight advantage since the AI is terrible at combat) while still allowing tech progression.
I find Epic to be the best compromise for slow progression without the sheer boredom of Marathon during peacetime.