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Frankenstein's castle is the 5th mission, actually. If you jump onto the top of one of the carriages you should get a life replenisher. There should be a Half Life Glyph in the area too.
The door needs a more powerful shotgun to be opened so nothing can be done about it just yet. Go into the door on the upper left and check the books on the shelf. Next, shoot the statue with your crossbow to reveal a crate. Exit through the door on the top level and swing across the room to a door which should be open now. After you kill some monsters here you should get a speed boost and upgraded shotgun ammo. head back to the library and blast the crate with the upgraded shotgun ammo to reveal an easter egg in the fireplace (sily place for that don't you think?).
With the egg retrieved backtrack and use the egg on the egg pedestal to unlock a challenge. Head back to the castle and use your shotgun 9now that you have the upgraded ammo) to blast open that door on the lower level. Check out the maps, books and generators you'll find and go through the blowing doors to an exterior garden.
Hope this helps you get through! 
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