Hi just curious to find out from you guys, when you attack with a stack of units, be it 2 units or 22 units, do you attack first with the lower XP units, or the higher XP units then clean up with the lower? I find I usually attack "from the right" ie. the lower XP, because I don't mind losing some, then the higher XP has an easier job cleaning up, and still survive. Also, how do you split up your siege attack? What defence do you wait until to finish bombard, then attack with the siege units? (BTW I hate the AI's choice of defenders, it usually chooses my medic unit to defend with!)

I don't know much about which units to attack first with, but I usually use my lowXP ones first.

For seige, I always bombard to 0%, then suicide 2-3 cats for collateral damage.

Usually the lower XPs will go in first unless there is another unit more suited to deal with the defender, ie. it has a Cover promotion if it's up against Archery units. Promote to units strengths (Swordmen make excellent City attackers with the Cover promotion) and suicide your catapults once you've bombarded the defences to zero. I use the higher XP unit to mop up so they gain even more XP and then upgrade them when the techs come along. If the lower XP units win, it's an added bonus. Some improvements require high level units like West Point which needs a level 5 unit. It's all planning ahead in most cases, perhaps once you get Long Bows and Knights throw them the bonus for dealing with Gunpowder units with the intention of upgrading them later.

Depends on the circumstances.

Normally I will attack with artillery units first (first those with the lowest experience and then the units with higher experience [or which I want to have a high chance to survive and gain experience]) but of course from the time of tanks onward my Artillery units (aside from bombers) will be Tanks/MBTs ;)

Aside from this it depends on the situation:
If there are some, few, tough defenders I will normally use the unit best suited for the job (because of its promotions) unless even the best unit will have chances of survival below 50 % (because all suitable untis have less modified strength than the strongest defender and/or are severely wounded)
in this case I will send in the most expendable units first to weaken the strongest defenders, so that the more important units might survive.

Remember there's a substantial advantage if the odds are in your favor, even slightly. Because of this, I check my strongest unit, and if it has an odds advantage, I attack with it. Otherwise, I attack with something weaker
(often a suicide catapult) first, the check again whether the odds are now
in my favor. If I'm out of catapults, I attack with a weaker, expendable unit,
which will often do enough damage so that my experience unit has an odds
advantage and can finish the job.

I would start with bombard units. You want to weaken a stack with collateral damage and finish with units specially able to take out the units in question.

Yes, but you don't want to waste bombard units when normal units can finish the job. Any bombard units you don't use can be used on the NEXT city.

Suicide bombard, balancing number of cats spent and left.
Then I go with the units having more chances to win. Out of them I prefer units which have little experience left to gain a level. I prefer 4/5 to 7/10 for instance

Thanks for the replies, the "odds" check is important, as well as saving your higher XP units. I must mention the "city attack" promotion is a lot better than any other for city capture, and sometimes using a unit without this, say in the early game before cats, is suicide and a waste. An interesting question is whether to promote your armours with collateral damage or city attack? It's a numbers game too, but I suspect one should spread these across stacks. A "by-the-way" with this is that when I played the Catherine-cottage game (and lost) I did not have religion and therefore none of those free XP gained from the civic - in future I think I prefer state religion and try other methods of appeasing other civs.