The Andalusian Mosque is the great monument of the eastern half of Fes el-Bali, founded in 859 — the same year as Al-Qarawiyyin across the river — and substantially rebuilt by the Merinids in the thirteenth and fourteenth centuries. Non-Muslims cannot enter, but the Merinid doorway on the north side is visible from the street: a deeply carved portal in carved cedar and stone, flanked by pillars, that is one of the finest pieces of Merinid decoration in the city.
The surrounding neighbourhood is significantly quieter than the zones around Al-Qarawiyyin. Visiting the Andalusian Mosque requires walking through the less-touristed eastern medina, which is itself the point. The lanes around it are where you see Fez operating as a real city rather than a museum.