Final answer:
The architectural characteristic first present in Harappa is the division into a residential lower city and an upper citadel with a tall and thick mud-brick wall, indicating sophisticated urban planning.
Step-by-step explanation:
An architectural characteristic first present at the urban center of Harappa includes the division of cities into two sections: a lower residential area and an upper citadel that served ceremonial and elite residential purposes. The citadel itself was surrounded by an impressive mud-brick wall that was both thick and tall, equipped with rectangular towers at intervals. This structure reflects the sophisticated urban planning and architectural developments similar to other ancient civilizations, emphasizing monumental, ceremonial, and administrative functions as significant aspects of Harappan culture.