Quote:
Rainbows take advantage of the way the combat system works where the leading wave of melee defenders will stop to engage the leading wave of attackers even if there is only a single attacker and a 1000 defenders or a single defender and 1000 attackers while the ranged units pound them mercilessly.
Ranged units, on the other-hand, ignore the layering if the opposing ranged units are in range, at which point they simply attack the opposing ranged units.
What this has to do with taking out wall fortifications is to send waves of warriors + rainbow to get the warriors closer to the walls before taking damage from the towers. This has the side effect of clearing out part of the opposing forces defensive rainbow if their gates are open.
Something like
5000 warriors
1000 Pikemen
1000 Swordsmen
1000 Archers
1000 Cavalry
1000 Cataphracts
What that does is take out: Trebuchets, Rolling Logs, Traps and Abatis so that the main attacking force doesn't start combat at max range.
Archer + rainbow is for taking out opposing armies such that each layer of the rainbow will stop the opposing army for one round allowing the archers/ballistae/catapults to shoot at them. In some cases archers, ballistae and catapults are layers for cavalry and cataphracts.
I don't 100% agree with all above, but probably some of the best explanations.