Imagine your army all wounded and tired after a battle, disorganised and without willingness to continue a fight after the slaughter they experienced just a few days back. Then suddenly they see defenders running at them in a surprise attack. They don't know it's just 1000 men, they can imagine it's 1 000 000 and they already have no will to fight whatsoever due to 0 morale after a lost assault...
They all run away or surrender immediately. Are happy they can rest now. After a while they will realize the stupidity of their surrender but it will come way too late to matter.
And on a more real life note, wtf is your cavalry doing not being with your main army? It was a big mistake to send them away. Mistake that would be forgiven in the old system but is punished severely with the new one so you avoid making it ever again.![]()
I seem to remember a reference to a battle in the Art of War in which a outnumbered force attacked the enemy during a sandstorm and won because the enemy thought that they were outnumbered.