Stronghold Lower Level is a mission in Syphon Filter. It continues from where 'Rhoemer's Stronghold' left off, and requires the player to complete most of the objectives from the previous level.

Eliminate Rhoemer's 9 scientists/ Administer antigen to 4 test subjects/ Find security key cards/ Find entrance to catacombs

The objectives are exactly the same as in the previous mission. Start by going down the slope and eliminte the guard that is at the bottom. Turn left and enter the room on your left. Administer the antigen to the test subject there and be prepared for an attack from above. Get back out when you are done and take your right.

You'll reach a larger hall. Do not go down for now, but keep going ahead instead. There is an enemy at the end of the balcony. Take him down and turn left. In the room up there is a scientist. He might be running towards you. If he does, just make sure you stand in his way and he'll get on his knees. Kill him.

Come back to the balcony. On the other side of the hall is an enemy. Shoot him down and use the slope to get to the lower floor. Take your left once you are down there and go towards the laboratory room in front. Another scientist will come running like the previous one. Eliminate him and go through the glass door in front of you. Climb on the boxes on your left and go through the tunnel like window in order to reach a second laboratory. Down there is a test subject. Give him the antigen and go through the glass door on your left. Climb on the boxes like you just did and go to the next room. You will thus reach a third lab room. In this one, there are a few enemies. The first one is right below you. Kill him and climb down to where he was. Behind the doors on your right is an armed scientist. Be cautious because he almost always keeps running, which makes him a tough target to hit. Once he is dead, leave this place through the doorway on the opposite side of the lab. Down the slope is a test subject. Give him the antigen and follow the corridor back to the large hall.

Right in front of you is a corridor in which you must go. As you turn the corner, you will see one or two enemy guards which will try to run away when they see you. Kill them and keep going until you reach another large hall, this one divided by a large arch above. Cross under it and go towards the corner to your right. Climb over the boxes and take the corridor there. Administer the antigen to the test subject you come across (objective completed/checkpoint).

You end up in a courtyard, be alert because a few enemies will attack you, most of them equiped with flak jackets. Make your way up to the other end of the area and climb the tall pile of boxes there in order to reach the balcony above. When you get to it, an enemy will attack you from behind, on the fortification. Take care of him and go in the corridor in front of you. It'll lead you back in the large hall with the arch. There are two guards in the balcony with you, the first one wearing a flak jacket. Besides, after you've taken a few steps into the balcony, a third enemy also equiped with a flak jacket will burst in from the same corridor you used. When they are all down, climb over the guardrail and into the arch in order to cross over to the other side. As you do, a guard will fire at you from the ground level. When you reach the balcony, go into the small room and elimiate the scientist there. He is armed, so again use caution. Take his security keycard (checkpoint).

Come back all the way to the outside balcony. An enemy will be waiting for you on the upper balcony on your right Take him down and climb to where he was. Destroy the stained-glass window and get inside. Turn right and kill the scientist on your right (he's armed too). Unlock the door to your right which leads to a corridor.

First take your left and you will come across another scientist, not armed this time. Take care of him and keep going until you reach a long laboratory filled with test tubes of all sort. Make sure you don't destroy one that contains a deadly gas. At the end of the room is a third scientist, armed with a .45. Kill him and pick up his security keycard (checkpoint). Immediately turn around and prepare for a quick firefight as an enemy will be coming at you. He may be trying to throw a grenade at you. Deal with him and come back to the door you previously opened (you will come across another normal enemy on the way).

Go past the door and you will see two guards standing at the other end of the corridor, each one protected with a flak jacket. Take them down and keep going until you reach a locked door. Use your keycard to unlock it and open it.

You end up on a balcony, on the second floor of a large rom. Enemies will attack you from all around, most of the wearing flak jackets, including an armed scientist. This is a tricky part, so you might want to use your K3G4 providing you haven't spoiled your ammo before. Once every one is down, keep going and take the corridor in the corner opposite of where you came in. You now reach the second floor of the rose chapel. Eliminate the enemies and, after a short communication from Markinson, make your way to the arches in the center of the room. Walk towards the large stained-glass window and there will be a cutscene.

You are on the ground level oustide the fortress. On the fortifications around are two enemies: one on each side of you. Kill them and climb down to the ground. A tunnel will lead you to the next area. You'll come across two enemies equipped with flak jacket on your way through. When you reach the second area, an enemy with a flak jacket will fire at you from a roof in front of you. A little to the left of him is another tunnel, but the entrance will be protected by another enemy with a flak jacket. 

This tunnel leads to a cemetery. Between the gravestones is yet another enemy wearing a flak jacket. The next tunnel is on the opposite side of the area. However, after you've turned the corner, you'll see two enemies above on the fortifications and one will throw grenades at you. You should keep running to safety in the tunnel will ignoring this last pair. In this tunnel you will come across the last scientist you need to take down (objective completed/ checkpoint). You will also have to face a few enemies, all wearing flak jackets.

After that you will come back inside the cathedral and you will go through already seen rooms, but this time you find yourself on the first floor. There will be a couple of enemies in the room with the large columns and, as you might be getting used to, they all wear flak jackets. Once you reach the chapel, take the slope down to the catacombs entrance.

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