Empires also earn revenue from taxes levied on each city for the first few turns or so, thus encouraging empires to expand their borders. Even allowing for the fact that this was the period of consolidation for empires, this seems too easy to do. Unless a neutral city happens to have money, which is quite unlikely, an empire swallows it up without a fight. Combat works simply enough and conflicts tend either to be short and sharp, unless a large enough force can get into a city under siege. Ownership of certain cards improves the chances.
Sieges are similar, but probabilities are much lower and thus tend to take longer.
All armies are more or less the same, the only difference being whether they are Imperial or mercenary forces. With navies, an empire has an interesting choice between Round Ships which cost more, need not end in port and can be used for explorations, and galleys, which are twice as good in combat. Thus, the details of combat itself are not all that interesting. What remains is the question of to what use to put the army. Or actually, the two armies as each player controls both an Imperial and a personal one.