Thanks j-menz. Is voice doubling when one of the voices is presented in octaves? As when the bass voice is in octaves to imitate an organ or something?
Generally, more voices = harder; faster = harder; more complex contrapuntal techniques = harder. How they balance out is largely personal. Initially, the interrelationship between the voices, hands and staves can create problems, as well.Added to that there can be issues of voice doubling. There can be more complex structures (eg double and triple fugues etc). There can also be fugues in different styles which more or less follow normal fugue "rules". There can also be atonal and serial fugues. These are more or less difficult depending on your familiarity with the style involved, but aren't really relevant here.