I was going to go all historical - an ideal way of doing things. But I decided to keep it as fun as possible and invent a scenario that would appeal to me - allowing me to have any units I liked but looking sort of correct. I'm back to my time as a young boy playing roundheads and cavaliers I suppose.
So a quick outline -
Sir William de Walloughby has decided to raise an army to fight for the King against Parliament. He is to raise a small force to hold the town of Wallingborough.