Ketchaoua mosque (pronounced 'Ketchowa') which is at the foot of the Casbah has quite an interesting history. It was mosque then cathedral and then later, mosque once again. Originally it was built by the Ottomans but was coverted into a cathedral during French colonial times; it was restored back to a mosque in 1962.
"The Ketchaoua mosque (Djamaa Ketchaoua جامع كتشاوة), at the foot of the Casbah, was before independence in 1962 the cathedral of St Philippe, itself made in 1845 from a mosque dating from 1612. " *
The mosque has some very beautiful architecture as can be seen in the photographs I managed to snap.
You can just see the mosque in the picture below; there are crowds of people because the street just near to the mosque is the site of a popular market selling dates, clothing and other items.