I'm missing something that should be obvious, and as I'm jumping around the rules I think I'm looking too hard for it. So please, someone just tell me where to look and what the significance of a level is in a city. In other words, why is it more difficult to attack a level 8 city over a level 5? Many thanks.
I was literally in the same situation not two hours ago. You'll find your answer in the main Rulebook page 6, Movement 5g.
Each colored dot represents one Enemy token of the same color.