It's possible this story relates to the fort (or indeed, it could relate to the Roman one of Cicucium, upriver). Whichever, you'd imagine the grain would be pretty manky. Which makes you wonder if it's a remembrance of a real discovery somewhere here, combined with the stories of the saint?
Whilst Cadoc studied at Llanspyddid, famine raged in the land, and the master and his pupils were put to straits for food. However, Cadoc observed a mouse carrying a grain of wheat. He succeeded in catching it, and borrowing a thread from a widow, tied it to the foot of the little creature and let it run; whereupon it darted into a hole. Cadoc dug on the spot, and discovered an underground chamber stored with grain. Such secret granaries were by no means uncommon, and are found in many ancient Welsh, Irish and Scottish forts. Or it may have been that one of the hypocausts that have been discovered at Y Gaer had been used as a storehouse for grain. On this supply the master and his pupils were able to live for some time.