Few other tricks:
- When scouting, look for other Civs' city connections outside their cultural boundaries. Pillage them. When done scouting, sit on top of it. Enemy workers will gather there and wander around.
- When AIs declare war on a city state that you want to protect (e.g. one you are allied with), send some units (e.g. scouts) and occupy the tiles surrounding the city state. This prevents/reduces the tiles AI can use to attack the cities. If they have a lot of naval force, occupy ocean tiles. AI units aimless wander around with only a few units attacking the city state at a time.
- Sell luxury resource even if you have one copy of if a city state is giving you another copy of it. It won't reduce your happiness.
- If you are at golden age and expect another great artist soon and have excess happiness, sell luxury resources (even if you have one copy of it). Extra happiness is useless during golden age, unless you have a social policy where half of your happiness is converted to culture.
- Buy expensive buildings, build cheap ones. Production cost/gold cost differ significantly between buildings. Expensive buildings (ones available at later tech) tend to have higher production/gold cost ratio. For example, medical labs cost 500 production or 1350 gold. So each gold is worth about 0.37 hammer. Granaries cost 60 production or 340 gold. Here you are only getting 0.18 hammers per gold. Exceptions, of course, exist as you would want to rush buy some key buildings.
- Sometimes after you capture a barbarian worker that belonged to a city state, you also find barb(s) next to where you captured the worker. Return the worker to the city state and allow barbs to recapture the worker. Repeat and gain massive influence points. If you happen to have couple of units near by, block worker path to return to the city state.
