The game normally automatically selects the best defender....except for when the
AI attacks. Apparently it can choose to wipe out specific units, such as weakened, higher promoted units in a stack first at will...I've seen it many times where the AI will attack my injured Legion/Horse armies and kill them off on the responding turn after attacking them, instead of the fully healed warrior army I have sent along for sacrificial cover.
The answer to how the the AI is taking the city with one combat? Overruns. When you can overrun the city's highest defense, all the other units automatically lose too and you take the city. Zulu can overrun at 3:1 instead of the normal 7:1 as one of their civ's unique abilities, which means that if they are sending an army (I assume Legion army) against your single archer defenders, they CAN overrun and take the city in one combat if they have the proper upgrades and/or your archer is unfortified/weak. Zulu Veteran Legion or Veteran horse armies attacking at 9 could overrun 3 defense archers easily, while with the Infiltration promo or a hill to attack from they could hit 12 power and overrun 4 power archers.
I don't know if they definitively can, but I haven't seen any evidence of it even in my hundreds of Deity games, that the AI will try to use this ability to cherry-pick the units to fight in a stack to take advantage of the overrun mechanic....when they cherry-pick it's always normal combats, not overruns that I've seen.... though I suppose it is theoretically possible. I must admit I haven't really tried to set up a scenario where the Zulu are able to abuse their overruns against me though....which is really the only feasible way an AI can ever overrun your units barring a huge tech disparity since fortified single Archers (3 defense) can normally defend greater than 1:7 against
even Knight armies (12 attack) without upgrades factored in (even then, fortified archers can STILL avoid being overrun by non-Zulu 18 power Vet Knight armies!)
Is this something I can take advantage of as well?
In a way, though it isn't very meaningful. Basically if you know the defense isn't very stalwart in the city/unit stack you are attacking compared to what you've brought. In this situation you would hit with your strongest attackers
last hoping to gain the overrun on single units or lower defense units and skip several more rounds of combat.
Some hypothetical examples where it could be useful:
-overrunning a settler's escort unit means you can still take the settler with the same unit on the same turn. You will lose ALL movement when capturing the settler though.
-Overrunning barbarian huts means you can take them out with a single unit in one turn. The unit actually kills each barb in succession but doesn't lose a move until it takes the hut, so it gains all the experience for the combats too. This tactic is actually critical to building a powerful rush early on Deity with horse armies, as you don't have time to build a barracks to get them to Veteran and every turn is precious if you're going to beat the ~2100BC archer army threshold! Note that for whatever reason, single warriors attacking high enough and even warrior armies at 4.5 (9 times the power, usually done with a hill or veteran) cannot overrun barb huts, but horses can!
-Attacking a stack of multiple Legion armies in the field guarded by one archer/pike army. Go any higher than Legions/Horses and the armies they form generally have enough defense to avoid overruns. Kill the defensive army first with one of your weaker attacking units, then hit the weaker defense Legions with your strongest army. If you can overrun one of them (which will be the auto-selected highest defender) you can kill them all in succession with a single attacker, after killing the defensive army. Since overrun is a 7:1 ratio for most civs, this requires a lot of advantage like more advanced units, naval support, veteran promotion, attacking from a hill, or the Guerilla promotion inside your borders.
-Saving turns when taking cities in high stakes attacks. This actually happens sometimes on Deity where you might be hitting a city with 2+ archer armies and some spare units inside with something like 3 horse armies, and failing the the attack would set you back horribly and force you into a drawn out game. Basically, save a strong attacker as long as you can and use it to overrun when the defending armies are dead and all that's left are weaker single units to instantly take the city. This can actually make a BIG difference if you are unlucky enough to have JUST too few attackers to take the city's last defender out, only to have the AI double rush units next turn (builds one through normal production, then instantly gold-rushes the other) and build another army.
-Similarly, you can overrun the weakest/last defender in a city to avoid a settled GP from preventing city capture (even when it's empty!!!). This aspect is highly annoying but is usually less break-or-bust compared to not taking out the city's last defender and getting a sudden army. Still, if you are doing something like trying for the fastest possible domination times, it saves turns.
All of these are much easier to do playing as the Zulu, as their overruns only require 3:1 or better ratio and not 7:1 or better power ratio.
