My main city is in Iron Age and easily defeated all the troops on the Continent Map to the end of LMA. Why? Because I camped for the last 2½ years I've been playing and built up my city.
My city production is over 400 fp per day, but actual production is way, way higher. Why? Because I built a lot of fp producing event buildings and leveled up all of the great buildings that produce fp (except InnoTower) to over level 60. I also built and leveled my Arc to above level 80 and donate to a lot of other peoples buildings. This has given me full sets of prints for every great building, even the ones I never plan to build, and I have hundreds of full sets for some buildings sitting in my inventory.
I don't have any goods buildings. Why? I built and leveled my Chateau Frontenac to level 100 and do hundreds of recurring quests every day, giving me thousands of goods daily as well as tons of blueprints and medals, and a few hundred fp daily.
I don't have any military buildings except a lone Rogue Den. Why? Because I built my Alcatraz great building and leveled it up. GE provides all the regular troops I need, Traz provides the Rogues. Also I don't lose many troops even though I'm approaching 20k fights because...
My attack is 347/287 and defense is 50/91. Why? because I leveled up (or in the process of leveling up) all of my military great buildings to 70+. I have also built lots of event buildings that give an attack or attack defense boost. My defensive stats are just a byproduct of buildings that either give fp or attack bonus. Having high attack statistics allows me to play extensively in Guild Battlegrounds and all the way to the end of GE giving me thousands more fp, and a lot of other stuff.
Because I built up my Arc and donate to other's great buildings extensively I was able to purchase all of the medal expansions a long time ago. Because I play to the end of GE every week I earn a lot of diamonds, which allowed me to purchase all the available diamond expansions without needing to spend any real money on them, as well as spending diamonds on some events without spending real money, and I still have over 10k diamonds sitting in my inventory for future age expansions. Traz and event buildings provide all my happiness. Event buildings provide all my population.
It is all one great big feedback loop.
Do you need to camp in an era as long as I have to be this successful? No, absolutely not, but not advancing eras too rapidly allows you time to build up all aspects of your city and game. I did so because I wanted to. I have other cities to experience the different ages. I have this one set up this way because that is what I want to do, not what anyone else has told me to do. Play your own game, but if you are having trouble and you don't consider other's advice and implement that which will help you play your game better (while still being your game), then you will inevitably flounder and fail.
And I did it as all as a basically free player.