[BTS] What eras to war in

I'm a prince level player and i want to get better. One of the things I am torn about is when to start a war (I always try to win war victories).

I don't know if you should do any medieval or classical warfare or just cavalry stomp everyone. it seems to me that its to many opponents t take out that late in order to win, but if i do any earlier it doesn't work.

So basically what I'm asking is which eras to go to war in. I understand it's circumstantial but give me general tips.

Thank you!