Call a tournament, or wait for a tournament to be called once you have gained enough land. When the jousting starts, lance the enemy's horse in the head, which will have you promptly thrown out of the tournament. All your land will be stripped. Now, raid your old castle (the one that was stripped), but do not acquire it! If this is done right, there should be a malfunction and you will have infinite money. You can now buy as many men as you want in your garrison.

While the game is loading, hold the K key until it finishes. You are then given 2048 knights for offense/defense.

When sending forth your campaign army, make sure to transfer all your troops in it and don't leave anything in your (home) castle. If your castle is being attacked, it is magically defended by the same amount of troops your campaign army is consisting of. You can transfer the remaining castle troops to your campaign army and effortlessly increase you army this way.