Other mistake: While the Trojan warning gongs ring, in the shot of the citizens when they run past the guards to return to the safety within the walls, there is something stuck to the left side of the camera (it looks like a long piece of hair), which flickers on the screen as the camera continues to pan in closer. (00:35:30)
Other mistake: There is a thin vertical line from the top of the screen to the bottom, that moves as the camera moves, just after the Trojans enter Apollo's temple when the Greeks run out of hiding to ambush the Trojans. It also appears in the first exterior shot, when Hector shows Andromache the secret escape route. (00:45:55 - 01:47:15)
Other mistake: After the battle outside Troy's walls, the Trojans rout the Greeks and Hector begins to ride back towards the city. In the final shot where Hector is riding away from the camera, if you look closely, you can see Greek soldiers standing among the Trojans, still equipped with their distinctive helmets/shields. (01:23:40)
Other mistake: During the first shot of the 1000 ships, oarsmen can be seen rowing, but their oars are not disturbing the water. Yet as they approach the beach, the shot is from the same distance and now splashes can be seen.
Other mistake: After the battle in front of the gate of Troy, the field is cleaned. This includes every scrap of flesh, every drop of blood, every footprint, and even some grass is replaced as is seen when Achilles rides up on his chariot to challenge Hector. This mistake cannot be explained by saying that the Trojans are shown cleaning the battlefield because it would take weeks even months to do that.
Other mistake: The umbrella used to shelter Paris and Helen during the parade in Troy is obviously modern, as it includes metal spokes like modern umbrellas.
Answer: They didn't know the Greeks weren't bringing cavalry. The Greeks did use them and sometimes brought them by sea. It was there to stop a cavalry charge and to break up fighting formations.
LorgSkyegon