I moved this over from Chaosti's Insane/Insane thread because it's too off-topic. Burress asked a good question, and I've done some delving, and the results are interesting/baffling...
Nick-Danger, how in the world does the AI get an army with 1000+ defense? I don't remember the damage formula exactly, but it doesn't seem like they could be damaged by any weapons.
Short answer -- no idea.
Longer, incomplete answer:
AI units have different stats in strategic view vs. when in tactical maps (above pics from tactical).
Here's 3 of the pictured units (from Choasit's thread) from strategic view (again, from my current insane/insane game):
Direfield Spearmen and Tradesteel riders (exact same):
Note the Direfield Spearmen have a base 591 defense in strat map (tradesteel riders too), where their tactical defense in my previous post is 596 and 997.
The numbers differ from strategic view to tactical view. For a particular Spikes of Krax, strategic defense:
Same unit in a tactical battle:
So, tactical has a 'defending' bonus, but there's more too. One would think the difference between the defense values would be 282.5 (25 x 11.3) but 566 - 259 = 307 so somewhere there's another 307 - 282.5 = 24.5 bonus in tactical from some unmentioned source (am I missing something obvious here?).
Anyhoo, these units can be killed without much difficulty, so I don't know how/what the high defense is doing. I don't remember the specific damage done to the 900+ defense units, but generally my ranged mage units seem to take all AI units down ~60-100 damage a shot (assuming no fire/ice resistance), with not much difference seen between the 200 defense and 900 defense units. Four of these units is my main means to kill these enemy armies:
...with 4 of these melee units to form a 'wall' should that be necessary:
...and my Sov just casts heal/slow and the occasional haste.
Spearmaidens don't do much damage to the high defense AI units, maybe 10-20, occasionally a bit more.
Better melee units, such as the two below, can do 40-90+ to moderate defense (~500 defense) AI units:
I decided to replay some old battles to see how much damage I did on high defense units. As noted above, defense changes from strategic to tactical. Here's a Spikes of Krax in strategic:
Same unit, at the start of tactical battle:
Defending bonus of +25 plus some unnamed bonus accounted for the increased defense.
Same unit, after my mages did 92, 63, and 58 damage:
Krax blood bonus of +8 somehow kicked in -- but not until after the third attack.
Same unit after a fourth attack did 77 damage:
Defending bonus lost -- the unit moved.
Regarding damage done to this unit by ranged fire/ice mage attacks (see above for stats), in this and several other battles:
vs defense 566: 92, 63, 58, 68, 81, 62, 63
vs defense 655: 77, 63, 59, 79
vs defense 357: 83
Possible small decrease in damage dealt at higher defense.
damage done vs other units of various defense:
vs defense 37: 86
vs defense 99: 83
vs defense 136: 97
Sample size is too small to draw all but anecdotal conclusions. The high defense seems to help a bit more vs melee units than the above ranged fire/ice mage attacks, but no numbers from testing so far so it's just me guessing.