An ecosystem could mean a lot of things, whether it be wildlife or a city. Imagine combining structures and unique AI to create whole cities of people and then having a war take place within this complex mess. Recreating some of the darker battles/massacres from the Crusades or Viking history would call for something like that.
This could also extend to things like disease. For instance, there were historical battles where a castle or city that was under siege would be attacked by their opponent using dead, infected bodies which would eventually get the people within sick.