Complex sentence: Landmarks are important for route-finding, especially where paths are indistinct (as on many Scottish hills), and he advises on what problems to look out for on more challenging paths (such as the "bad step" on Crib Goch).
Split into simpler sentences: Landmarks are important for route-finding, especially where paths are indistinct (as on many Scottish hills), and he advises on important features to look for on the paths he describes. He also warns of problems be aware about on more challenging paths (such as the "bad step" on the traverse of Crib Goch).