There are three sizes. Diplomacy, even in the earliest stages, seems to be more important, with your reputation bobbing up and down somewhere between the extremes of Spotless and Atrocious according to your penchant for honouring your promises or sneak attacks. The pace of the game is as in Civilisation, ie 20 years per turn up to 0 AD, then 10 years until the late Middle Ages, etc. The number of government types is increased, however, by the addition of Communism and Fundamentalism. On the other hand Monarchy seems to fall apart very quickly after the Middle Ages.
Democracy seems a bit easier than it was in Civilisation.
You can slip quite a few of these into your opponents' cities and subvert them by bribery, steal their technology, sabotage their assets or even plant nuclear devices. This last is a major pain as you see most of your 2,000 plus years of agricultural work undone by the sudden incursion of deserts, jungles and swamps, leading to a rise in Hunger. My knowledgeable friends have made a couple of observations about the mechanics of the game which I would agree with. One is that a combination of Howitzers and Railways can be rather unbalanced as you can use the unlimited movement of railways to move your big guns to pound cities with unrealistic speed. The other is that, at the lower levels at least, Spies are also rather devastating, especially if you like the idea of planting nuclear devices.