Keep your army upkeep under your tax income. This is a good gauge to tell you how many more units to recruit.

Plan ahead. What I mean is you don't have to always be building in settlements that will probably become a castle. (example... Byzantine... there is no use building infantry buidlings in castles in the early game because your cities produce the same spearman.

Going off of Byzantine example. You have to go easy on building buildings with this faction because you start out with low money and have more settlements than other factions and Constantinople is already an advanced city which requires expensive buildings.

Quickly recuit merchants and grab all 4 resources near constantinple (cotton?). Your merchants will level up quickly and as soon as 1 of them gets 4 points, head west for the gold lot near Ragusa. There are always tons of enemy merchants in that vicinity and your high level merchant has some easy pickins.