The Chellah is a walled ruin of a town that lies in a short walk from the current medina of Rabat.
It sits on the banks of the Bou Regreg and was built by the Romans in around 40 AD. It was one of the two main Roman naval ports on the Atlantic (the other being Mogador) and lay on one of the two Roman roads in Morocco that led down from near modern Tangier. It contained a principal Roman way (the Decumanus Maximus), a temple (to Jupiter), a forum and a triumphal arch.
The Romans however lost control of the town in around 250 AD and it was abandoned in 1154 when the inhabitants moved to Salé (on the Atlantic, across the river from modern Rabat city). From then the Almohad dynasty (which ruled until the early 13th century) used it as a cemetery (necropolis) and in the 14th century the Marinid dynasty added the defensive wall and towers (that we now see at around 6m high – a defence against Spanish attack), a mosque, a medersa (Islamic theological school), further tombs, the main gate and other monuments.
It was designated a World heritage Site in 2012, is the most popular tourist attraction in Rabat and houses both Roman ruins and a medieval Muslim necropolis.
(Source : Naturally Morocco)