How/when do spies appear?

[sav]

Can someone explain simply how and when spies appear? I can't work out how to generate them... the Civilopedia entry I read was brief, to say the least.

Loving the expansion so far - so much more to do in the early game!

 

[anandus]

You'll get your first spy as soon as someone hits renaissance. And after you get another spy for every new era.

 

[sav]

You'll get your first spy as soon as someone hits renaissance. And after you get another spy for every new era.

Is it that simple? I thought there'd be some kind of way you could up your espionage output.