Rainbow said:
well this is my usual strategy for city improvements
1.walls
2.barracks
3.granary
4.aqueduct
5.temple
6.marketplace
I suggest that problem#1 for many players is having a "my usual strategy". The greatest beauty I see in this game is that there is no usual best strategy. To my mind the best strategy is always dramatically affected by what's available in the start region on any given map. It is also strongly affected by your goal, further affecting the optimum opening sequence.
Having said that I would suggest that your usual strategy would be improved by:
1) Never build walls. They're nearly the worst bang-for-the-buck of any possible improvement. (Colosseums might be worse.) If you are finding that walls are useful then you need to work on your "best defense is a good offense" thinking, and should read lots of the strategy and war academy articles on this site. Once you get rolling walls will be a waste of time.
2) Do not build barracks until you are in a position where they matter. I.e. a position where you will soon be building military units in that town. Building barracks before that time just results in a maintenance cost with no corresponding benefit.
3) Build granaries only in towns where they gain more food per turn than building a settler. Sending out a settler and founding a new town gains two food/turn when the settler founds a town. So only build granaries if they gain more than that, either because you're out of expansion room and you have a lot of growth still possible (implies a lot of other things such as many luxuries - this will not usually be true), or a town has a food bonus it is working such as cattle, wheat, or game.
4) Don't build aqueducts until you need them. Earlier is a waste of shields and of maintenance income. When possible avoid the need for aqueducts by settling on rivers and lakes.
5) Temples are only occasionally worthwhile. Build them only where either:
a) Border expansion is a priority and you are not a scientific Civ, or
b) You are going for a cultural victory
6) Treat Marketplaces as a high priority in core cities, i.e. in cities which are not totally corrupt. As long as you control or can trade for at least 3 luxuries, a Marketplace will gain as much happiness as a temple, and will gain gold too! And if you have 4 or more luxuries the Marketplace gains more happiness than a temple. The Marketplace is a very valuable improvement. If you are not at war it should usually rate higher than the other five you list.
7) Libraries should usually come before many of the improvements listed. Libraries give more culture than temples and increase your research capability dramatically.