Interesting - all goes to show how different people interpret things in different ways. Of those who have commented, it seems that I am 1:3 in the minority, even though it transpired that my 'interpretation' was actually the correct one!For what it's worth, I thought three walls were involved too.
As I wrote this morning,for just a few seconds I wondered about that as well, but when I looked at the second diagram, I immediately 'decided' that there were actually only too walls, since it seemed very unlikley that the OP would very clearly indicate the position of one corner but giving no indication at all of a second one (had there been a second one).