Although perhaps uninspiring at first glance, this is in fact the only Almoravid building to have survived the fervent renovation spree undertaken by the Almohads who succeeded them. The Almoravid Koubba, or Koubba Ba'adiyn, is a small, domed building that may have been an ablutions annex to the original Ben Youssef Mosque. The dome's design was the model for the classic shapes and motifs used in future Moroccan design. The Koubba was built well below today's ground level, and until an excavation project in the 1950s had been covered as a result of the numerous rebuildings of the mosque. You now have to walk down two flights of stairs to reach the interior where, besides the fascinating dome and ceiling design, you can also see a water cistern set into the floor and remains of the fountains used by the faithful for performing ablutions before prayer. Photographs of the excavation project are on display inside the ticket room.