Official historiography says that Great Wall of China is built "across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe." — (Wikipedia)
Eurasian Steppe is located North from Chinese states, so the fortifications supposed to face North, toward the place where possible invaders may come from.
However, looking at pictures of Great Wall, I notice many places where the fortifications rather look like bi-directional, e.g. the parapet seems to be convenient to aid defense from both directions, to North and to South, merely equally.
Here's what I mean:
(Left: Unidirectional; the parapet is built for defense on one side; right: Bidirectional; the parapet is built for deference to both sides; can shoot arrows from parapet slits in both directions)
It is not only about walls itself. Some towers also look like they are built to assist fire in both directions.
Question: Is there a decent explanation of this phenomenon?
Disclaimer. I'm not a military expert, so answers like "your question comes from a false premise, because…" are also greatly appreciated.