Fadhloun Mosque is one of Djerba's best architectural stops away from the beach hotels. It is a low, white, fortified rural mosque, built with the practical island logic of a place that had to serve as school, refuge, storehouse, and lookout as much as prayer hall.
The details are modest but fascinating: buttressed walls, roof access, a small minaret once useful for signalling, and ancillary rooms around the complex. It explains Djerba better than another resort strip does, especially if you are interested in how island communities protected themselves.