Also - Zygmund mention not fighting within the city... I have read this kind of thing historically, and for example in that movie Troy, it showed the defending army leaving the city to fight in front of it. Any idea why the heck anyone would do this? I mean, if you have city walls and all, use them.
My idea is that Empire armies, especially during civil wars, seldom need to defend their capitals. Instead, battles are fought closer to the 'front', maybe defending smaller cities. This doesn't rule out getting besieged in your capital city. I guess it must have happened to every Empire capital city at one point or another, and these battles are the grand ones that are being remembered and written alot. But I imagine it's quite rare.
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As to why the Greeks seem to have given battle outside of their city walls before the Persian and Peloponnese Wars, I can give two reasons, both of which were heavily debated at the time and are heavily debated still among the historians:
1) Battles were in some way more tribal and more honorable. There was an agreement when and where to conduct the battle, there was less slaughter after the battle, and usually a peace afterwards. Stories that tell of a heroes' fight in front of the armies might actually be true, and echo modern 19th century accounts of tribes encountered by Europeans. It's a way to spare people and resources, and maybe fight another day. If a city was besieged and subjugated, the whole population was often either sold to slavery or slaughtered. So there was a reason to give fight outside and accept the peace terms instead of getting besieged and fight to the bitter end. Archaic cities could be relatively small too, with little space for the entire population and storages to keep them nourished during a siege.
2) Although there were cities, the population of the cities was often tightly tied to the surrounding countryside. Archaic cities were city-states, and the 'state' was much more than the city only. Actually, cities might have a predominantly slave or foreign population, whereas most of the citizens lived in the countryside. If an enemy army was pillaging the countryside, most citizens would want to protect their fields & houses in the rural hinterland. The city-dwellers were thus highly motivated to leave the protection of the walls and to give battle to the enemy.
Both 1) and 2) started to change already in the Greek/Hellenistic times. The Empire of the Old World is in many ways completely different to archaic Mediterranean and Near East.
To start with, the population of the Empire cities usually forms a juridically independent body, not tied to the surrounding countryside. This follows the medieval pattern of locally independent communities and cities. Think of the citizens of Visby closing the gates and watching how the Danish army slaughtered the Gotland peasants outside of the city walls in 1381, and then making separate peace with the invader (practically paying the Danish mercenaries by allowing the troops to pillage the city). The archaic Greeks would not have understood why such was possible - i.e. why the urban and rural population wouldn't have fought as one body.
-Z