Armies in Civ 3 = taking 3 units and turning them into one with the possibility of multiple attacks (if movement points allowed it). Armies in Civ 4 = sending units of different types into enemy territory to take a single city. Result = More attacks and better result (unless you count a single unit [with two attack and triple (on the first attack) strength] as being better that 3 individual units attacking after a proper bombard + attack w/ collateral damage using siege units.