The city has been taken over by a hostile army. Build up your army with heavy military hardware and take back the streets - try not to destroy the city while you are at it!
Click troops to select them. Then use WASD or Cursor Keys to control the troop. Aim with the mouse, Click to shoot. Move the mouse to the edges of the screen to scroll around and take a look at what's ahead. Some units need to stop to be able to shoot (namely the Bazooka guy and the Artillery truck)