The song below seems to have a definite medieval flavour to it, but what causes this? Is it the guitar or vocal melodies? What scale/mode aspects give this feel?
I'll give this a shot. Some elements contributing to a Medieval sound are
Minor-key, modal melody (I think it is Dorian)
Melody is catchy and song-like and follows a resolution pattern that resembles old drinking songs or sailor songs.
The 6/8 rhythm also contributes to a drinking-song feel.
Harmonic voice-leading features prominent parallel motion (parallel fifths jump out)
The strummed instrument is troubadour-like
I agree. In particular, it reminds me of the Nibelungenlied (as sung by Knud Seckel). I hear three clear similarities:
- the melody, which uses the same incomplete scale and is actually quite similar
- the harmonization, which stresses fifths and does not use the fuller chords we're accustomed to from later European music
- the instrumentation, which is sparse and improvised