Choose your team, then press the corresponding key on the key pad. With 1 being the team on the left, 2 the middle team, and 3 the team on the right. Keep the key pressed down and move the pointer under the door and click the LEFT BUTTON four times, then the RIGHT BUTTON four times. The border should flash to let you know if it worked.
While being attacked, press 'ESC' key repeatedly. This will reduce attacker's score, inflicting less damage on your soldiers.