"One advantage I found for England is to immediately take Scotland and hold it. It may take a while for the population to settle down but the cheap Clansman are worth it. Start to specialize Wales as an archer province so that you are ready for longbowmen. When you have enough men to hold Ireland buy it off and get your Gallowgass (sp?) production up. With some good spear in back, Irish gallows, Scottish clansmen, welsh longbowmen, irritating light horse (turcoploes or jinettes) and if available heavy horse you can really rout any army you face.
I found that clansmen and gallowglass are the best for holding desert states as long as you have spear to hide behind. Even with exhausted spear those two units have held off Muslim armies that outnumber them even 10-1. Always ensure you have some kind of archer unit behind your spear to wear down advancing forces. If you can, bribe a muslim army to get their desert friendly forces then while the stack is blinking, launch a crusade to the province and you will have some heavy spear and horse (with loyal captains) ready to go with your newly acquired desert force.

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When the Golden Horde shows up and you have been campaigning in the muslim provinces, take either Georgia or Syria+Antioch as your northern borders. The Syria+Antioch tactic allows for the mongols to wear down the armies in their path (usually you are are at war with them if you are campaigning there already) and wear themselves down before they get to you.
I like bribing stacks of mongols when they show up. Their horse archers are fantastic to use, and the mongol warriors make great back up archers/skirmishers or garrison troops. You can get to the point where they only have 3-4 stacks left and then are not that hard to take out with spear and heavy cavalry. Unless you are going to put most of your resources into an army to hold off their first showing, cordon them off until A.) you have taken the rest of the board or B.) they have worn themselves too slim." - Puhallo

"Place 4 ot 5 units of archers in double line formation one behind the other. Surround these with 7 or 8 units of spearmen/fuedal men at arms/halbediers etc. in a diamond formation so that the archers are protected on every side. Have your cavalry units to the rear ready to outflank the enemy when they have committed themselves to an attack. Make sure you click the 'hold formation' button before the battle. When the enemy attack they will face an impenetrable wall of spears and will mill around looking for a weak spot. Never use peasants in this defensive formation as they will always be attacked and will route easily. Your archers can then fire away at their leisure, cutting the enemy to pieces. As soon as the enemy general joins the fray, get all of your archers to fire at him. In the meantime, circle your cavalry around the back and hit them from the rear. Once the enemy general is dead, the others will break pretty soon. If in doubt, pile everything you have into the enemy general and take him out." - ScareySlug

"Dont invade scotland or Wales, send in a emissery and bribe the them, saves alot of Units and money and wasted years." - Whitefox