At the crucial moment, Zhao Kuo personally led the 1000 man cavalry to block off Shanhai City's 2nd regiment.
From the 2nd regiment's perspective, the cavalry of the Zhao army were like clouds, they seemed like they were beside you but you just couldn't hit them. When you drew back your blade, your enemy just charged towards you like a well oiled machine. It was really difficult to defend against this quick and vicious cavalry force.
Luckily with Er'Lai, the 2nd regiment was able to get into a defensive formation and it was enough to protect themselves.
After all, as the player troops had a limited amount of men, even though they could create a hole in the 40 thousand Zhao troops, they were unable to break through to assist the 2nd regiment.
The Zhao army cavalry immediately retaliated after they regained their senses, using their numbers advantage to box up the enemy troops.