A very nice piece. Patina.
Obverse: bust of the emperor in laurel wreath, armor and paludamentum, to the left, in the rim legend:
CONSTANTINVS IVN NOB C;
Reverse: camp gate, surrounded by legend:
PROVIDEN-TIAE CAESS, in segment ANT;
Bronze, diameter 18.5 mm, weight 3.4, axis 12 h.