The enemy can come from only one direction - the gate- and the enemy can only send so many people though the gate that you can take his army apart piece meal. Engaging at the gate, limits the ability of the enemy to flank you and the enemy ability to bring all the men he could to fight with to bear if there was no bottle neck.
So, a smaller unit of troops could fight a large number of troops and win. Huscarles, Joms, Beserkers, and other high level units especially.
Fighting isn't about winning, it's about depriving your enemy of all options except to lose.
*edit*
... That is just asking to be my sig.
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