Well here's a few tips
ALWAYS get trade rights with every neutral faction around you.
When there is a wall or castle upgrade available, alwaysbuild that first
Build economic buildings everywhere, I generally go with this order:
roads
farms
mines(if available)
ports(if available)
merchant ports(if available)
markets
other buildings (merchant banks etc)
While trade buildings don't seem to increase your income by much when looking at the city you build them in, remember they also benefit cities around them
While it may be tempting to set all taxes to v high, its actually better to make them low. This will allow you to garrison less units to keep order, and will grow population faster allowing for higher level settlements and more taxpayers when you really need the money. If you at anytime go bankrupt or desperately need more troops, setting all taxes to very high should be enough to get you out of
.
Also always take advantage of the free upkeep slots. Every castle or city can keep a certain number of units they can produce garrisoned inside them for free(right click the castle/wall building to see how many). Forts also keep (I think 3) units of any kind for no upkeep.
Don't keep unnecessary garrisons in central settlements. At the start, take as many rebel cities as soon as possible. You should be able to muster enough units from your starting settlements and armies not to require wasting money on buying more