Build up units and send them over to the enemy base in waves to try and defeat their main base unit.
Use 'A' & 'D' keys or <- -> keys or drag minimap to scroll the map. you can sell or repair your tower by clicking the sell or repair button on the bottom-right of the screen. 3 command buttons (on the bottom-left): Defend, Alert and Attack, respectively so you can easily command all units by clicking one of them. Press the SHIFT key to view all units' life bars. Hold the CTRL key and click on a friendly unit to make a force attack. Hold the CTRL key and click on ground to let your units move to that location to guard and attack enemies they face. More instructions and tutorials are available in the game.