I remember a siege in which I fought off a half-stack WRE army - with at 5 comitatenses cohorts... I had 2 keels and a unit of levies, as well as my faction leader.

Put the grunts into a shield wall, which stopped the Romans dead, and had my general flying about doing multiple flanking attacks - that shield wall was quite literally a wall. Valour wasn't even all that high either - only a couple bronze chevrons.