You will obtain some fairly useful information in this stage. When you approach door A, the soldier who spoke to you from the communication room in Area 9 will invite you to step inside the door. When you enter, you will see an enemy behind the spikes. He will taunt you, but you cannot attack him. Instead, find a way to swing over the spikes and visit him. Even if you climb up to the ledge above him, he will talk. Once motivated, he will tell you that there are hidden underground passages running beneath Neutral Area 14 and this area, as well as between Neutral Area 15 and Neutral Area 18. Sure enough, when you look at the map, there will now be lines running between those areas.
In the Japanese version, the enemy can be attacked. But instead of doing so, you are to hit him with the grappling hook a few times, and he will spill the secret out. Additionally, if you have the time of hitting him 255 times, he will also praise you and unlock all of your guns.
Last, but not least, head over to door B and step inside. A man standing on the center platform will inform you that they've located Super Joe, and that he is being held in a P.O.W. camp in Area 7. This should be your next destination, but before you visit, it is worth taking the time to explore the underground passages that were pointed out. If you explore them, you will find the Iron Helmet item which, like the Pendant, is capable of deflecting shots. However, unlike the Pendant, it works on the first three shots that hit you.