Goofs

When Lin comes up the stairs to say goodbye to William, there is a guard several stairs lower than where Lin stops. At different camera angles, the guard is right near Lin and he changes up and down one stair.

When William jumps off the wall during ring of fire, he initially was a little too far from the chain to slide down it.

Machinery inside the Great Wall is 'powered' by water wheels, upon which the water splashes prettily rather than forcefully driving them. And even worse, there can be no stream of water flowing above these mechanisms to provide a power source.

The "drone" Tao Tei's eye placement does not conduce to its position in the horde. As a protector and hunter for the queen, it would need the predator's forward facing eyes, rather than side eye placement, to allow for binocular or stereoscopic vision. This would allow the monster to see and judge depth especially during the great-height wall battles against the Nameless Order. Predators need this depth perception to track and pursue prey. Its current design suggests that IT could be the prey.

On numerous occasions, the archers are ordered to 'fire' their arrows. Until the invention of fire arms, the English word used regarding archery would be 'loose' (loose the arrow/s).

That is a loose translation of the ancient Chinese language.

The Tao Tei are "deafened" and incapacitated by magnetic fields. In that case, they shouldn't be able to survive at all within Earth's magnetic field which is far stronger than a small permanent magnet.

The Earth's magnet field only seems big but compared to a magnet it is extremely weak. It has ~25 micro Tesla (equator) to 70 micro Tesla (poles). A small fridge magnet can have 50 milli Tesla, classic horseshoe magnet 100 milli Tesla (4000x stronger than the field at the equator). Therefore a mangetic lump of the size used in the film is definitely way stronger than the Earth's magnetic field.

When William demonstrates his skill with the bow in the great dining room. Then he shoots 2+2+1. But you only see three arrows. One to stop the rotation of the bowl and two that the bowl balances on in the stone pillar.

In addition, the timing of when the arrows reach the bowl are not successively fast enough to match the actual release of the arrows.

He only shoots 3 arrows. He holds all 3 arrows in the right hand, firing them one after another. Can be clearly seen in slow motion.

When we're first seeing Lin Mae jumping off of the wall to fight, during her jump in which she has a spear in each hand and does a back-flip, one of the spears can be seen phasing through the rope when the two should have collided.

The film shows a manned balloon being used to hunt the dragons. The first known aeronautical balloon was flown in France in 1783, nearly 700 years later.

While the Montgolfier balloon was the first in Europe, it is possible that one may have been flown centuries earlier in this fantasy/legend.