I started a game of Civ 5 BNW with the mod that makes the USA completely broken (every important unit is upgraded, start with all 4 base techs unlocked, start with 3 settlers, 3 workers and a super strong scout) to have some conquest fun (although I forgot to actually enable the conquest victory, or any victory for that matter). I didn't have any other factions on my continent (180X96 earth with true starting locations), so I could tech up in absolute peace and quiet.
So I teched up to Peacemaker (giant death robot of the US) and started conquering the other civs: Egypt, Russia, Korea, Venice and Greece. I saved Korea for last because they were the farthest away and I would have to cross the entirety of Asia to get there. I was planning on taking over all the city states on my way there, but that actually wasn't feasible. While the cities of all other factions had extremely low defense (20-30 next to their shield icon), all the city states had stats in the mid-to high 100's, like 160-180. They could go toe to toe with my peacemakers and come out on top.
Why were those city states so damn strong? Is there a hidden effect for this somewhere?