The main quarters can be found in front of the main courtyard (M2.5), and the entrance (in the basement) is protected by two soldiers. However, if you get to the second floor, you will be able to walk above the basement, and find a chain that can be used to climb down (you can drag away the guards by throwing rocks).
Approach the marked crate and in the main quarter to refill your supplies. You will obtain explosive charges that can be used to destroy weak walls and blow up the main door of the castle...
When you enter the next courtyard located in the north, by the church, you will end up in the middle of the battle between the Nazis and Partisans. Your hero will suggest to support the Partisans but you may as well focus on your own tasks - one way or another, you need to get climb the stairs. A good idea would be to collect a Panzerfaust below the first stairs leading from the church to the courtyard (M2.1).
A good sniping position is the main round window in the church (M2.7). If you have not done it yet, you should go to the graveyard and collect the document from the mausoleum (see above chapter on searching the graveyard).
Reach the top of the stairs and approach the main gate of the castle (M2.9). Plant the explosive charge and activate it for the second time to set time detonator. Otherwise, you would have to shoot at it.