A4: You will arrive on the roof through the elevator in this room, but you won't make it very far without the bomb blast suit. Big Boss will call you on the transceiver and inform you that you need to suit in order to get past the "window barrier" when it should actually read "wind barrier". If you attempt to step on to the roof without one, and sheer wall of wind will shove you back to the elevator doors. If you don't have the suit, you'll need to explore building 4 a little more thoroughly. If you have the suit, put it on to step out onto the roof. Once you pass through the small alcove by the elevator, you can safely remove the suit. From here, you can go west or south. Unfortunately, there's nothing of interest to the south, and you will reach a dead-end at C4, and be forced to turn back. Head west to room A3.
A large wall blocks you from proceeding any lower, so you must return north to room A4, and then head west to room A3.
A3: Three guards stand stationary in this room, looking around. To avoid detection, you should only venture to cross the room when the guards aren't looking your way. Attack them or sneak by them. Then continue heading west to room A2.
A2: Two guards stand motionless at the top of this room. You should be able to avoid detection by staying along the bottom of the room. From here, you can head south across a suspension bridge, or head west. There is still some things of interest to find before you cross the bridge, so head west to room A1, but stay along the bottom of the room, or you may be detected by a guard when you arrive.
A1: One guard patrols the room. Attack him and make your way to the left side of the room. Then proceed south to room B1.
B1: Surprisingly, you won't find any guards here, or any threat of any kind for that matter, just a door on the left side of the room. Through the door is a room with ammo inside. Exit and enter the room as many times as necessary to obtain a full supply of ammunition. Then continue south to room C1 through the left passage (the right one dead-ends).
C1: Once again, no guards will threaten you as you make your way down and around to the door in the room. Open it with card 2, and you'll find a prisoner to rescue. Exit the room and return outside.
At this point, a wall prevents you from going any further south. You must backtrack to room A2 where you could access the suspension bridge by going south to B2.
B2: When you enter this space, you will be presented with a narrow suspension bridge that crosses directly over the courtyard. If you mess up here and step over the sides of the bridge, you will fall to your death below. It is important to watch the movement of the bridge and step accordingly. From the top, favor the left side of the bridge and carefully move down when the bridge panels are in front of you. By the time you reach the middle, you should move Snake more to the left, this time favoring the right side of the bridge panels. From the middle, you can make your way down to the bottom portion of the bridge, and continue south to C2.
C2: This room is just the mirror image of the suspension bridge above. Favor the right side of the bridge from the top to the middle, edging right as you continue down, until you are about half way and can start favoring the left side of the bridge. Make sure your health is good, and proceed to room D2 below.
D2: The southern portion is guarded by a soldier that wears a jet pack. However, despite having jet packs, these soldiers can't actually fly. When you arrive in the room, it's possible to fire you gun south and shoot the guard from across the room before he even notices you. From here, you can head east or west, but you're going to need an item from the room to the west, so head to D1 first.
D1: Two guards are stationary in this room. If either guard notices you, the one closest to the door will pull the trigger on the electric floor. At that point, you must escape to safety and use a remote control missile to knock out the panel and make the floor traversable again. However, it is difficult but possible to knock out the guards before they notice you. You can easily remove the top guard by attacking him while hugging the upper wall. The lower guard will see you as soon as you step on to the lowest portion of the room. He will glance upwards on occasion, but very briefly. When he does this, it is your chance to slip around the wall and attack him before he sees you. However you get through the room, your goal is to reach the door in the lower left corner. It opens with card 3, and inside you will find the land mine detector, an item which will become very useful in the near future. Collect the detector, and step back outside. Return east to D2 and run straight across to D3, shooting the lone guard along the way.
D3: The guards won't notice you if you hug the wall as you enter this room. Then you can attack the closest guard and run past the second. Rush straight ahead to D4.
D4: When you arrive in this room, you will be confronted by two soldiers who man turrets in front of two doors. They are known as Twin Shot.
First approach the right door with card 3 activated… but to your dismay, you will discover that Dr. Pettrovich has already been moved from this cell. Exit the room, and open the left door with card 2. Inside is a prisoner who will tell you that Dr. Pettrovich has been moved to building 2 when you rescue him. He will also tell you that building 2 is directly north of building 1. Armed with this new information, it's time to make your way out of this building and on to the next.
At this time, your immediate objective is to return to the elevator, take it to the first floor, and exit the building via the way you recently entered (through the back door with card 4.) From there, you must follow the outdoor path to building 2. However, before you leave building 1, you may want to visit the truck on the first floor that contained land mines, as well as the room on the third floor that had remote control missiles, and stock up. Otherwise you may find yourself having to come back to building 1 later in the game.