Your goal is to survive as long as you can while wave after wave of enemies attack you. Do not let your command center die.
As you progress, you will be confronted more and more with trucks that carry turrets on them. Foot Soldiers will also spawn at various times and they do not follow the path. As you play you will learn where they spawn from.
How to Play
Arrow keys move the screen. You may also click on the minimap to move the screen.
Use the tabed menu in the bottom right to select units to build. If you have a unit selected, you may click anywhere on the HUD that is empty to deselect. Clicking on the middle tab opens up upgrade options. The third tab is intentionally left blank.
You can zoom in or out using the plus and minus keys on the num-pad or the plus or minus buttons on the HUD.
Some units can take more of a beating than others. Also, certain upgrades affect certain turrets better than others. Having the right balance is key to surviving the longest. Also note that all units fire upon the closest enemy.