If you're looking at the overhead map, this happened in the lower left corner. If you're in the castle, head up the stairs towards the hole in the wall and out of the castle along the ridge towards the stone cottages. Thread the needle between the first two cottages and there's a curved wall on your left.
I died three times there. The fourth time I circled around from the flank and noticed bullets appeared tobe coming out of the stone wall. I looked all over for a shooter, but couldn't see anyone. Messed with the map in Local mode but couldn't see any legit way to get behind that curved wall.
Anyone else experience this yet?