well i lets say that 400 can do the job against boxed mountain ,and u general and if u have 2x larger army with good composition in comparing with defender.And if he has fortification 6 .I remember that i used 400 against boxed F6 stone collectors for f7.Dont take this number for granted.Also having 12800 heavy swordsman in every round of fortress siege should be standard
this work only if the defender doesn't have any "defender" (army in field battle)