Once you use the thermal goggles to open the door, you will enter a large open room. Note: Under the normal difficulty setting the keycode is "1436" or "1463". To the right will be a sentry gun. Run straight forward and out of its range, then face the gun. To the left of it will be the computer. Go to it and "Disable IFF" (the circle will be empty), or just disable it. Then, jump up on the shelf you see to your upper right. Pull yourself up, jump on a jar, then climb up where the banister is broken. Go to the sentry gun's computer and make the circle next to "Disable IFF" empty. Duck behind the gun then watch as two guards walk out and get shot. The last sentry gun at the top of the steps on the opposite side of the room is not a problem. Look behind you (when you are behind the sentry gun that just shot the two guards) to find a pipe. Jump up on it and shimmy across. You will see a guard and a colonel. Get back to where you are not visible by the colonel. He will let the guard through the door. When he has his back to you, slowly creep up behind him. Make sure you do not knock him out when you jump off the pipe. Force him to let you through the retinal scanner door and continue on with the mission.