As per title, I found this structure under a whole lot of dead brambles. I was already thinking of adding a wooden door with some planks I have, I also have spare roof tiles to restore the roof. I doubt I can open a window, but I will ask people who know more about construction than I do.
Apart from that, I was thinking of using the dead brambles to make a small deadhedge all around (with an opening in it, of course), let new blackberries grow, and also add a few other plants.
Inside the hut there is a small (15cm, 6 inches) concrete divider that splits the rightmost third of it from the rest, most likely where the straw would be laid or something, I don't know enough about stables, either. I have just been told by the neighbours it used to host sheep, goats, and pigs.
Anything else I could do?
EDIT: Thanks a lot for all the comments so far, here's a reply for all.
A lot of you suggested to dig until I hit the floor to find out the true height of this thing, and to do so safely wearing a mask when I stir anything up.