When the charge bubble in the bottom right corner is full, click an attacker to kill them. When an attacker gets to the castle, it will slowly drain your castle's health. The higher your defense level the less damage you will take. You can upgrade your kill charge speed, regeneration amounts, and castle's defense level for the following prices: Kill speed: 40$ Castle defense: 50$ Regeneration speed: 60$
Please give ideas for updates in comments. Also please post what you don't understand about the game in the comments so I can put it in the instructions.