Rhoemer's Stronghold is a mission in Syphon Filter. It involves Markinson, through Gabe, shutting down Rhoemer's operations in a cathedral. Logan is told to, among other things, terminate all the scientific personnel and administer a vaccine to test subjects infected with the virus.
You start this level on the roof of the cathedral. Take a few steps foward and climb down on the arche on your right. You'll see an enemy down below. Use your sniper rifle to take him out easily. Keep walking carefully and you will be attacked by a guard from behind a stained-glass window. Kill him and make your way to that same window in order to enter the building.
Take the corridor on your right and shoot the first scientist you see ahead. There is a doorway on your right which leads into a small room. Enter and you will receive a transmission from Markinson. There are two test subjects in here: locate them and give them a shot of your antigen (press Triangle or select the antigen and 'attack' the subjects with it).
Come back in the corridor and turn right. Turn the corner and you will see a monk guarding a large wooden door. Shoot him and keep going (ignore the door, you need a keycard first). Ahead is a dark small room which has supplies in it. There might also be a guard there, but for some reasons he's most often not there (if there was a guard by the large door before, this one won't be there). To the right of this small room is the second scientist. Eliminate him. There may be a guard with him too. Go in their direction and you will come to an intersection.
First go straight ahead, towards the laboratory. Behind the glass door is a test subject. Give him the antigen and come back to the intersection. You will then be attacked by enemies in the corridor on your left. Deal with them and engage in that corridor. You will come across a second laboratory room on your left. This time there is a guard in front of the glass door. Kill him and go inside the chamber in order to administer the antigen to the test subject there.
Come back in the corridor and go left to the third laboratory. Behind the glass doors are three scientists. Eliminate them and pick the keycard that one of them is carrying (checkpoint).
Back in the corridor, four enemies will attack you. Eliminate them and make your way back to the large door. A new guard will be in front of it. Kill him and go to the switch to unlock the door. Then go press triangle in front of the door itself to open it. In the room behind it, there's a pile of boxes. You can easily climb over them. Once at the top, enemies will start shooting you from far ahead. He's actually on the other side of the cathedral, in a small alcove. Snipe him and go out through the window.
Turn left on the balcony and eliminate the guard in front of the stained-glass window on your left. Do the same with the other one on the balcony with you. Another enemy is located on the other side of the cathedral, like earlier. Now there are four stained-glass windows on your left. The first one is the one you came through. Behind the second window is a room with a scientist in it. Kill him and come back on the balcony. There is nothing in the fourth one but a K3G4 rifle (that I strongly suggest you pick up). Finally, behind the third window is the exit you are looking for.
Fallow the corridor and you will catch up a scientist walking towards a guard. Take them both out and keep going until you see an inclination on your right. Above it is a test subject. Go to him and give him a shot of the antigen, but be ready to fight as two enemies will shut your exit. When you're done with them, come back in the corridor and turn right. At the end is a scientist protected by two guards. All three wear flak jacket, but notice the chandelier above their head. Eliminate them and take the security card from the scientist (checkpoint).
The door you can unlock was just before the last group of enemy. You now enter a library. ALL enemies in their wear flak jacket. The K3G4 might come in handy here, although it's quite possible to do without. In the back of the room is a box on which you can climb to reach the surrounding balcony. Three enemies are on that balcony. Eliminate them and go around the room. At the other end of the balcony is a bookcase on which you can climb. Destroy the glass stained window and go outside. There will be a monk on the arch. Kill him and cross to the other side. While doing so another enemy will shoot at you from your right. When you are on the other side, enter back inside the cathedral.
Below you is a guard that has not seen you yet. Use that to eliminate him and climb down to where he was. On your left is a glass door, and behind it is a guard standing behind a counter. He wears a flak jacket, so be cautious. Kill him and go beyond the glass door. There is also a test subject in a corner on your left. Give him the antigen and climb down on your left. An enemy is waiting at the bottom. After you've taken care of him, fallow the corridor up to an intersection. You will then see a scientist running by you. You can fallow him but stay alert as there is a guard on your left. Kill them both and take the key card from the scientist (objective completed/checkpoint).
Climb over the boxes and eliminate the enemy below (he has a flak jacket). When you climb down, two more enemies will attack you from behind. Kill them a go over the boxes on your left. An enemy is standing on the other side, and he wears a flak jacket too. Shoot him and climb over the last set of boxes. A scientist is about the go around the corner in front of you. Eliminate him (objective completed/checkpoint).
++++++++++++++ + Hint: K3G4 + ++++++++++++++ There is a dark room on your right with a peak made out of boxes. On top of it is a K3G4. As you enter the room, a guard will fall from above. Kill him. And as you start climbing the boxes, a second one will attack from behind.
Back to main guide ~~~~~~~~~~~~~~~~~~ Climb the incline on your right and you'll reach what looks like an attic. There will be three enemies there, all wearing flak jackets. They are tough to deal with because they move a lot and will somewhat try to corner you. Climb down on the other side of the room and open the door using the switch on the left wall. Open the door and kill the guard on your right. Exit through the stained-glass window.
++++++++++++++ + Hint: M-79 + ++++++++++++++ Notice that the last door you opened prevented you from going left (it was blocking the corridor). Once you're outside, you can actually get back inside behind that door through the second stained-glass window. You will then find a box containing an M-79.
Back to main guide ~~~~~~~~~~~~~~~~~~ Fallow the central arch until you reach the other side of the cathedral. Destroy the stained-glass window and climb inside. An enemy wearing a flak jacket will attack you from above a wall of boxes. Take him out and go right. One last guard will be hidden behind a column on your right. He too is equipped with a flak jacket. Nearby is a small elevator on which you must go to end the level.