i take an onager or two with me as the brutii.

currently against gaul and such, i take one, so i can invade-knock over some walls- and take a city in 1 round.

and im using scorps and onagers against scythia, as my roman army is fairly ill-equipped to fight large number of HA and strong cavalry, i plunk away at them with my scorpions and onagers while rounding them up with equites.

however, their dreadfully slow in the campaign map. something you have to weigh out