the cavalry should have been used on the flanks (or one flank).
the catapults should have been firing continuously to deplete enemy numbers (hindi kasi dapat sumugod yung cavalry)
all strategy gamers KNOW to put ranged troops at the rear.
the wooden spikes should have been IN FRONT of the unsullied. with their spears and the wooden spikes slowing down the dead ...
basically, it was in the script to decimate the dothraki and unsullied because there is a golden company enemy yet to be fought
-professional war movie watcher
There is light cavalry and heavy cavalry, each with its own unique use and attributes.
The humans did put the heavy cavalry in the flanks. The Knights of the Vale were with Brienne and Jaime. Dismounted though, as these knights would be better off using their armor as added protection as a holding element vs a fast and agile enemy.
Dothraki are light cavalry at best (very similar to Mongols, Huns, Scythians, etc). Light cavalry is typically at the front of the line leading a charge (or in this case, a very costly feint), particularly at enemy infantry NOT in an entrenched position or lacking anti-cavalry weapons like pikes (i.e. exactly like wights).
Contrary to popular belief by war movie watchers, the use of light cavalry in this episode was textbook.
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I do agree that they should've used the trebuchets to send more Greek fire into the enemy, but they had to allow the Dothraki to retreat (which they probably expected, as this attack was to draw the NK's main contingent in).
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As I've said, the humans' strategy was not to outlast the Night King's army but to draw him out into pitched battle and try to get lucky.
This isn't Helm's Deep where, if they could withstand the siege, they survive and win. They had to use their forces the best possible way.