the army and the upkeep

for a big and strong army woat soldiers you must have

Just speaking in defense terms, stick with Phalanx(24 gp/hr upkeep) and later Steam Giants(68 gp/hr upkeep). That have the best cost to defense ratios.

Yes, each unit has an upgrade cost which you must keep in mind, otherwise you'll go broke and won't be able to grow your colonies at all. Or worse your army will desert you!

Last don't forget the upgrade powers of the workshop, the defense and offense powers are very beneficial.