A cavalry fort in the middle of your city affords quick response to all sides. Also, use the overlays to see which areas are supplied with water, education, etc. Right-click on buildings, especially housing, to see how to improve an area. As your housing evolves, supply the citizens with pottery, oil, and furniture either by making it yourself or trading for it. If your income starts to drop, halt imports for a time but continue to export.
Prosperity depends on turning a profit and having elite housing structures.
It is critical that you have enough forums close to elite housing so that the rich pay their taxes. Also pay close attention to the employment level. High unemployment will cause prosperity to fall. Caesar always has need of oil. If you need farmland for food instead of olives, you can either import food or import oil, or replace a food farm with an olive farm for just long enough to stockpile a supply of oil and dispatch it to Rome.