Unit 1 - Level 4 - Lost Installation (Secret Level)
Exit this room and turn right to enter another room with plenty of guards. There is some ammo and Armor Shards lying around but proceed up the steps to the large room with the tower erected in the middle (enter from either side). Watch out for guards, Enforcers, and Flyers hovering above you. Walk down (or jump, depending which door you entered) and collect the power-ups on dry land and in the shallow water. Step on the platform to take you to the top of the tower and press the computer screen to lower the bridge, Walk across and collect your prize - a Super Shotgun. Your primary objective is now complete.
