Verge
Let's hug it out
The mechanics of combat in Civ IV play strongly against the zerg type rushes, unless the Zerged unit can do collateral damage ( that is completely unbalanced in Civ IV, with firaxis showing little imagination to solve it ( just disallowing siege of killing in BtS and making it extremely vulnerable to mounted units of that era ... not exactly elegant ( and to say the truth not even a real solution... but that is another issue ) ) ), due to the way that the diference in strength is magnified by the combat formula ( more details in the War academy article " Combat explained" ). If you want to do a zerg rush in Civ IV, just dust out the vanilla or warlords disk and mass catapults....
And by mid-game, with smart placement of production cities and a robust economy, you can build enough of any unit to zerg-rush fairly cost-effectively.