Cover the walls in dark colors or natural materials to form a base for your Gothic medieval castle decor. Mixing textures and colors can also work. For example, you could paint most of the walls blood red, apply a wallpaper mural showing a Gothic cathedral to an accent wall, and put in a stone wall or fireplace that suggests the wall of a medieval keep.
Arrange heavy furnishings made of wood, leather and wrought iron throughout the space you are decorating. Some furnishings that would work with a Gothic medieval castle look include a black leather couch, a wooden pew from a demolished church, a rough-hewn wooden door with primitive-style iron hinges, and a wrought iron bed with twisting, vaguely sinister bedposts.
Dress the windows and install lighting fixtures to accentuate the Gothic medieval castle mood. For example, apply "stained glass" treatments to a main window and swath the others in velvet curtains. Install an ornate black candelabra or chandelier. Line the walls with faux torches lit with LED candles.
Add Gothic, medieval and castle-inspired accessories throughout the space, such as wood-framed Victorian-era portraits, a self-designed coat of arms banner, mounted trophy animal heads (or paper mache facsimiles), and atmospheric stacks of Gothic horror novels.