Okidokey, if I'm to make my first topic here, it may as well be useful.

First off, destroy the enemy troops that start off outside the castle.

Then, have to cannon that starts with a peasant and a spearman unit beside it blast a hole in the wall directly ahead of it. Have your army move over to behind that cannon by hugging the edge of the map. Have your other cannon destroy arrow towers etc. When the breach is made, charge in.

Usually, the enemy will not reinforce the section of the castle you are attacking. Remember, it's only the section the keep is in that you need to hold. If the English decide to reinforce that section by coming through the gate, you are screwed. If the enemy decide to walk around the castle to get to you, then you have plenty of time, but you may want to detatch a unit or two to slow them down.