▁designated ▁in ▁ins cri ptions ▁as ▁a ▁numer us , ▁it ▁appears ▁to ▁have ▁been ▁struct ured ▁as ▁a ▁regular ▁mill i ary ▁( i . e . ▁double - str ength ) ▁al a ▁of ▁the ▁A ux ilia , ▁under ▁the ▁command ▁of ▁a ▁trib un us ▁milit um , ▁who ▁probably ▁reported ▁to ▁the ▁Pra ef ect us ▁Pra et orio , ▁the ▁command ant ▁of ▁the ▁Guard . ▁Initial ly , ▁it ▁probably ▁contained ▁ 7 2 0 ▁hors emen , ▁divided ▁into ▁ 2 4 ▁tur ma e , ▁or ▁squad rons , ▁of ▁ 3 0 ▁men ▁each . ▁Num bers ▁rose ▁to ▁around ▁ 1 , 0 0 0 ▁under ▁Had rian ▁( r . ▁ 1 1 7 - 3 8 ) ▁and ▁the ▁regiment ▁was ▁expanded ▁to ▁some ▁ 2 , 0 0 0 ▁horse ▁in ▁the ▁early ▁ 3 rd ▁century ▁by ▁the ▁em peror ▁Sept im ius ▁Sever us ▁( ru led ▁AD ▁ 1 9 7 - 2 1 1 ). ▁ ▁Their ▁home - base ▁was ▁in ▁Rome , ▁and ▁their ▁permanent ▁camp ▁was ▁on ▁the ▁C ael ian ▁Hill . ▁A ▁second , ▁supp lement ary , ▁camp ▁was ▁built , ▁also ▁on ▁the ▁C ael ian , ▁when ▁the ▁unit ▁was ▁expanded ▁under ▁Sept im ius ▁Sever us ; ▁this ▁has ▁been ▁exc av ated ▁under ne ath ▁the ▁Bas il ica ▁of ▁San ▁Giovanni ▁in ▁Later ano , ▁Rome ▁( see ▁Cast ra ▁Nova ▁equ it um