Calculating the odds with multiple units is extremely difficult, as you have to allow for defender promotions, attacker retreats etc. Sirp has written a utility which does this, but (at least on the version I have) this can't cope with large numbers. The calculations become outrageous. I therefore use a programme which simulates taking a town lots of times, using the same combat rules as the game. The program is available as a zipped executable jar ( so you would need java) in the utilities subforum: combat The 82% figure for 6 vet archers v 3 fort reg spear is based on 1 million simulated assaults on a town without walls, taking it 81.6% of the time, losing on average 2.1 archers in the attack.