Pre BTS I would have an army composed of city defenders, 4-5 stack defenders, and then a giant pile of siege units to take cities. They can get CR and do collateral damage; what more do you need to take cities?
Post-BTS, things have changed. Usually my armies are composed half siege, maybe a few more (never more than 2/3) and the remainder split between city raiders and stack defenders, with some city defenders tagging along to guard my spoils. For example, in the middle ages I'll have an offensive stack with 10-12 trebuchets, 4-5 catapults (which I started building at Construction, before getting Engineering) and 8-10 macemen, plus ~2 pikes (more if my opponent is HA/Knight heavy), and a crossbowman or two. I'll add a few longbows; one or two for each city I'm planning on keeping.
Usually I can throw together an army like this with 10-30 dedicated turns of army construction in all my cities, on epic. I don't often do that, though. On monarch, I'll have one or two cities always on military unit production. They'll build the siege and CR units, then as I build up to war my other cities will build my stack defenders and longbows.
When I attack a city I'll move up first, then destroy the city defenses. Then I'll check the odds on my best CR siege weapon. If it's over ~75%, I'll start with my best units and work my way down, attacking with as many siege units as possible then working my way through my CR melee units. After I'm done with those, I'll use my stack defenders, trying to line up my last attack with either a utility stack defender (shock/combat mace) or a city defender, then move one or two city defenders in to guard the city, viewing the stack as more valuable than the city. The next turn, I'll move my stack into the city and heal if it'll be faster to do that, or leave them where they are and heal with the medic. I just calculate based on mousing over fortify until healed where they are, versus holding shift and issuing a move order to the city, then mousing over fortify until healed to check which will take longer.