Does anyone know any good ones? I want to build up a good collection of Medieval acoustic instrumental songs.

A good op always goes first...

I got nothing :P
Medieval sort of encompasses a lot of different thoughts. I mean, there was the medieval period of music; and some of that music doesn't sound like what you are probably thinking. I would personally suggest "Many Moons Ago" by Empyrium. It's got multiple acoustic lines, but the primary melody is nice and fun to play. By extension, a lot of neo-folk has that sort of medieval feeling as well.
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Into infinite obscurity and Crimson towers by Dissection. Learned them, trying to find more myself