It looks the size and shape of a hen house to me. Sure was a solid one though, they must have had a good supply of stones. Ususally there would be a small door to let the hens out during the day. Nothing left inside to give you a clue? If it's facing South, it may have been a "hot house" to grow things to extend the growing season. The stones would hold in some of the heat from the day. It's hard to tell, but is should make a very nice hen house.
