Your skill with the ballista is very important. Practice so that you can shoot it quickly and accurately. Make sure to zoom in if you need to get headshots.
Power-ups
Preview the next level before purchasing power-ups. Some levels restrict your ballista, troops or spells and spending gold to improve them may be a waste of money.
Sir Gareth
Use Sir Gareth sparingly, as he is only available for a limited time. He is very effective at eliminating enemy ground forces because he is controlled directly. Use him only in times of great need, as when when the enemy has your flag or during the final push at your enemy's castle.