Now that I think about it, if your able to raise a peasant army, with good terrain tactics/ect, you might have a chance to at least give the Invaders more losses than they would otherwise recieve.
Still, no matter what you are being invaded with, having a peasant army hastily recruited would be better than no army at all.
Consider this. Lets assume that all the Weapons you make are 7 attack, and all Armors you make are 10 defense. So you have limited troops because it takes a long time to train for those weapons, or something, or your focused on CHampions and Quests. Or magic. Lets assume you are focused on Champions/Quests. Your city might have 3 Uber-CHampions. THen lets say your Military Heavy opponent invades with an army of 2,000 troops of 10 HP, 5 attack, and 5 defense.
You then have about 10,000 peasants you can equip with 1 HP, 7 Attack, and 10 Defense. (out of an 80,000 pop city)
On pure numbers, its 20,000 HP, 5 attack, 5 defense, vs 10,000 HP, 7 attack, 10 defense. Who will win? Using Pure numbers, the veteran army will obviously win, but using those 3 Uber CHampions, plus Terrain Tactics, City Defense tactics, as well as other useful abilities on the tactical map, you *might* be able to pull a Win or a Draw.
Don't Underestimate Peasant armies just because of their 1 HP. Of course ... that raises the question of how they got so many weapons in the first place ... but this is all hypothetical. (the *reason* for excess weapons was that the Nation's Training times seemed VERY expensive compared to equipment costs ... meaning their Armory Industry was probably rather high)