In this excerpt from the novel "The Mysteries of Udolfo" written by Ann Radcliffe, we see how the author develops the theme of fear. She is doing that mostly through the atmosphere - the surroundings around the protagonist are scary and dark; there are tall trees and woods that instill fear in her. She specifically says in the excerpt that these surroundings "awakened terrific images in her mind," meaning that she was scared of what might happen.
Other images include "the deep tone of the portal bell," which even increases her fear and anxiety about the whole situation. The building itself is quite scary, looking ancient and dreary which only makes her feel even more frightened.
The examples above are directly stated in the text to show her fear but the atmosphere itself makes even the reader feel uncomfortable with its unsettling nature. It is both directly stated and implied based on the atmosphere that the protagonist is quite frightened.
