I have to say that I don't know much about heavy metal, but this sounds like what I imagine to be death metal. I did a bit of searching on that premise, and I found one site that classifies (I think) this band as "Epic/Medieval Black Metal."
I do know a bit about medieval music, although I'm no expert, and this sounds almost nothing like medieval music to me. There is the modality, as you note, but that's about it. I therefore suspect that the answer to your question ...
why does this exactly sound so medieval?
... is that you have formed your own cultural associations with this style of music and the medieval period. You're obviously not the only one with this association, but I haven't yet found much in the way of authentic medieval elements in this music. I'll try to give it a closer listen tomorrow and edit this answer accordingly.
I tried imagining some medieval music played by distorted electric guitars, and it didn't sound much like this. The melodic development is different, for example, and the pitches change more actively.
I would suggest having a listen to some recordings of actual medieval music. Ask yourself whether there are any common elements that you might cite to rebut my assertion that this music doesn't sound medieval (after all, I may just be overwhelmed by the electric guitars and thereby blinded to some aspects of the piece that I should be taking into account). If you can find these elements, but you are still not satisfied that you understand what it is about this music that seems medieval to you, ask another question, post a comment under this answer, or edit your question to clarify.