The interior of the castellum is not well understood because excavations in the modern city centre are limited and fragmented. Only one major building, known as Building J, is sufficiently documented. Recent research identifies it as a double horreum, or granary for storing agricultural surplus.
This storage complex may have been central to the fort’s purpose. It would have collected and redistributed grain from the region’s productive agricultural hinterland to supply both local populations and military sites along the frontier.