Well, it may not be totally medieval...but it has been around since the 16th centuary. You can find info on it here- La Folia

La Folia chord progression

i-V7-i-VIIb-IIIb-VIIb-i-V7

I discovered the progression today while reading on the internet. I liked the sound so much, I thought why not write a bass line with it. Enjoy.