Go up fighting through large rooms filled with knights. Use your newly aqcuired Ainkurn Sword to do a little more damage and take some blood. Continue up until you come upon a door that opens up into a courtyard. Kill the knights and head through the other door. Kill the Tremere in the little church area of the building.
More knights emerge from pews in the chapel.
Eventually you'll come into the main entranceway of the fortress. There is a large group of knights lead by the Dark Knight. The main doors open to the Inner Stradt. Go to the East Ring Strasse and take the Tremere amulet to Orvus. He will enchant it to let you enter the Haus de Hexe.