When defending in a siege, heavily outnumbered.
Take your generals bodyguard, redirect them out another gate and try to sneak them behind enemy lines.
Postion them right behind a unit of heavy infanteries, but do it subtle so he doesnt turn around in advance.
When you are at about 100m/yards away, charge them. Make sure your units are lined as wide as possible and don't let them leave holes in your line as you charge.
If done correctly, your cavlerie will charge right throught his line of unsuspecting heavy infantrie like butter, losing noone yourself and being able to just runback towards your own gate as it is in that direction where you where charging.
Rinse and repeat while your enemy is busy taking your wall down.