To your right, stairs lead into the main part of this level. Before you follow them, check all the rooms to the left and straight ahead. You'll have to fight knights and Tremere. The four rooms can be accessed from the main corridor so take it one at a time. When you're finished, head back to the stairs.
Knights guard the hallways here.
Follow the chamber to the left. In the area behind the mural, you see a map of the city and the Tremere and Tuetonic leaders conspirating planning out a campaign. Kill them and then continue your course, opposite the archway where you entered in a large chapel. When you enter you'll be attacked by a group of knights. Kill them and approach the altar.