Job Specialization

The Jobs that citizens in the city specialized in were, such things as:

Workers and Artisans.
Military
Priests
Royalty (King)
Enslaved
Craftsman (same as Artisans)

These people were very good at their jobs. Another specialization of job in this empire were scribes.
This can be backed by the fact that there were extensive numbers of clay tablets that have been found.
Also, the alphabet was easy to learn, and therefore it was easier for more people to learn.