Build towers and upgrade them to prevent the enemies from reaching your castle. To build a tower select the tower icon in the bottom left corner. This will allow you to view a list of available towers. Towers cost resources which can be viewed on the top right. Upgrading towers is also very important, to upgrade a tower simply select a tower that has already been build and look to the lower right. A tool-tip will pop up, and if you have enough resources, select the visible upgrade icon. To change perspective, move the camera using the joystick in the bottom right corner.