Question :
                                            
                                            
                                            
                                                What is the semi-circular structure attached to the Holy Kabah?
                                            
                                            
                                            The Answer : 
                                            
                                                The semi-circular structure attached to the Holy Kabah is Hijr Al-Kabah (is a low wall originally part of the Kabah) and passing behind it is an obligation while making Tawaf because it is a part of the Holy Kabah. And Allah Knows Best.