Try building the basic castle needs, such as a Marketplace, Mercenary Post, Granary, Stockpiles, and Industry Buildings, then save there. It is easier to start at that point, in case your placement of some walls or towers do not work well.
Pause the game whenever you can to save time. It is very important to build faster than the enemy.
Keep your money making resources (Food, Rocks, Iron, Pitch, or Weapons) protected, or put walls around them to keep them safe. Capitalize on them whenever you can.
Create layers of walls around or in front of important towers and other targets that the enemy likes to knock down.
Horse archers are very useful in this game, especially when aggressing enemy castles with Mongonels to help put big holes in enemy walls and a few swordsman as guard dogs.
By immediately placing a group of Woodmans Huts around a enemy base, then putting them to sleep, you will block the area and prevent him from building Barracks or a Mercenary Camp. It costs a lot of Wood, but it means they cannot make any troops and therefore very easy to defeat.
Note: This does not work on the pig.
When attacking a castle, concentrate on the enemy Barracks. Once destroyed, try to block it from being rebuilt by placing troops over its remains. Again, it makes the enemy easier to defeat without it producing more troops.
The Ballista is good at holding positions far from the security of your home base. Get a group of about eight built, with approximately twenty Archers and stick a couple Trebuchets in the middle of them. The Trebuchets can attack the enemy castle well protected. If needed, you can always take the six Engineers from them to make three more Ballista when the enemy counterattacks. Ballista are also great at taking out enemy troops on the castle walls, especially fire throwers.
On several levels there is very little farm land, but you always find at least one resource available (Stone or Iron). Concentrate on the mining and sell it at the market for Food.
Have both Archers and Crossbowmen on Towers (five each) for a good defense. Add a Ballista on the Tower to make it better as they have a huge range and can even hit attacking Trebuchets.
To protect farms, etc. outside your walls, build a Tower and a Gate next to each other. This allows you to put Archers in and have them protected from enemy soldiers entering the Tower. You could also destroy stairs to the tower once they are in, as an other option.
Pikemen are the best troops due to the combination of speed, defense and attack. Knights are far too slow to attack with, but make great defenders.
In the most difficult levels when the enemy comes at you very early, try to set up a trap. Build strong walls around your land except in one area. Leave an opening and have as many troops as possible covering that area. The enemy often ignore the undefended walls and go for your "weak spot'" Instead, they walk in to a wall of Ballista, Archer, and crossbow fire; and a few Knights for good measure. Do not forget your Lord is also a superb fighter that also can be used.
It is all a matter of timing. Organize waves of attacking troops, attack with one group, then send in reinforcements. When blocking the enemy from rebuilding its Barracks, your troops will not last long. While they do the blocking, have other sets attacking.