The fortress is the only building left that stands between the barbarians and the people of your kingdom. They are attacking in waves and you need to put the huge cannon to use. Aim and fire it repeatedly at the enemy troops in the effort to keep them from destroying your walls. Also with the money you earn for each kill you will be able to buy upgrades. If you do so, you will be able to hire men to fight for you in the field. Each one will cost you but it will surely help in the heat of the battle. Upgrade your health and offense systems too if you want to survive the siege in this defense game.