1. Use your SP points wisely - worthwhile policies generating/modifying happiness has been spread out over 3-4 different trees - Piety, Honor, Order...
2. For a wide empire with a lot of annexed cities, the Forbidden Palace is
huge. For a +20 cities empire (own/annexed/puppet cities mix), the wonder built or captured, can easily generate happiness in the +30-40 range as it did in my latest Rome game. Notre Dame is great as well.
3. Always puppet conqured cities and when you have a happiness surplus, annex those worth annexing with regards to growth/production etc. Build/buy a courthouse in these as the very first thing. Conqured capitols are usually the first puppets you should annex. Build/buy colluseums everywhere and build the Circus Maximus wonder as well.
4. The AI plants cities
everywhere on Immortal/Deity. If you conquer any of these fringe/garbage cities, raze them immediately - they will never become worthwhile. If you are granted a bunch of AI garbage cities in a peacy treaty settlement, do likewise. Accept and raze them right away.
It's certainly doable to have a huge empire and positive happiness - also on harder levels, but you
absolutely have to plan for it. If you just go mad with conquring cities with little planning of how to administer your empire, you'll usually find yourself with a huge but fatally crippled empire with no growth, weak production and combat penalties for your units. Not recommended.
