One defensive strategy I always use is to build my town and leave the land that is adjacent to the ocean alone. So when the Kings Army land I get the bonues of the terrain as an ambush. The only bad thing is that the AI/europeans likes to slash and burn all the trees & plow around their towns, so when you take over their towns.....the King likes to attack those towns first. So I like to capture an enemy town and then abandon it. It only works if they don't have a stockade, fort etc.
I like to leave a mixed assortment of armies guarding my cities. Example: If your Continental Army loses, send in your Criminal/Dragoons and they MIGHT just win. :bleh: