Why is there a sacred Muslim symbol in the tomb of the Virgin Mary?

What does this symbol tell us about the complex history of Jerusalem?

In Jerusalem in the 11th Century, Muslims fought to re-take the city that had been conquered by Christian Crusaders several decades before. They began to tear down walls and churches but instead of dismantling Mary’s tomb, the Muslims carved a mihrab* in the wall. 

Why is there a Muslim Mihrab in the tomb of the Virgin Mary?

*A Mihrab is a semi-circular niche in a wall that indicates the direction of Mecca and hence the direction that Muslims should face when praying.