The walled city of Salé lies on the northern banks of the Oued Bou Regreg. Although nowadays it's effectively a suburb of Rabat, in the past Salé has at times been the more favored of the two. The settlement was first established in the 10th century as an alternative to the aging Sala Colonia across the river. In the 13th century, the ruling Merenid dynasty built the city's walls, which can still be seen today, in response to raids by Spanish looters. The Merenids also constructed a canal that passed through the main entrance gate, Bab Mrisa. The canal, no longer visible, offered secure access for merchant ships to sail right into the city, and as a result, Salé grew in stature to become one of the most important ports on Morocco's Atlantic coastline. Its stature grew even more during the 17th and 18th centuries, along with that of its twin sister across the river, when the two cities benefited from an influx of Muslim refugees from southern Spain, and the ensuing activities of the infamous Sallee Rovers. The Bab Mrisa canal was especially useful to the shallow-keeled vessels used by the pirates.
Once the renegade cities returned to central rule and European interest became focused on Rabat, Salé quickly fell from favor. The 1957 construction of the bridge, Pont Moulay Hassan, connecting the two only hastened Salé's decline as many of its citizens, called Slawis, shifted to the newer suburbs on the Rabat side of the river. Those who have stayed have kept their independent streak and "tribal" sense of belonging, however, and Slawis are considered among the most pious and conservative in the country. Some of the first independence demonstrations took place here, and Slawis are well represented in the upper echelons of government and the palace. Wandering around Salé's compact medina is a great half-day's excursion from Rabat, including the boat ride across the river that the city historically relied upon for its existence.
In the far western reach of Salé's maze of streets and lanes is the Almohad-era Grand Mosque. Constructed between 1163 and 1184, and one of the oldest in the country, the mosque sits in the top end of town, surrounded by a concentration of mansions and other religious monuments. Although non-Muslims can only see the mosque's stepped, main entrance and tall minaret, entrance into its adjacent theological college, or medersa, is open to all (rue Ras ash-Shajara, aka rue de la Grand Mosque; 10dh adults, 3dh ages 5-12; daily 9am-noon and 2:30-6:30pm). The medersa is a Merenid construction, built by Sultan Abou el Hassan in the 14th century and recently meticulously restored by the government. Similar to sister medersas in Fes and Meknes, the medersa follows a basic plan of a central courtyard opening onto a prayer hall, with upper floors of cell-like rooms for the students. The interior craftsmanship is superb, with zellij, stucco, and cedar woodwork covering the ground-floor walls. From the flat rooftop there's a great view looking over Salé toward Le Tour Hassan and the Kasbah des Oudaïas. The annual Moussem of Sidi Abdallah ibn Hassoun is one of the more important in the country, and sees pilgrims paying homage to the patron saint of Salé.
Your best options for getting across the river from Rabat to Salé are by boat or grand taxi. At the time of writing, boat taxis had ceased operating between the two riverbanks due to a major redevelopment of the riverside promenades on both sides. However, this popular and cheap (only 1dh per person) public transport should be running again by mid-2010; the boats leave when full from an area opposite the medina's Bab el Bahr in Rabat. Grands taxis for Salé depart from the rank next to Hotel Bou Regreg, arriving at another rank opposite Salé's Bab Mrisa -- an unusually high entrance due to its original accommodations of merchant vessels.