Tangier is a major harbour in Morocco and the point to cross to Spain/Gibraltar.
Fés is one of the King´s cities in the kingdom of Morocco. Known for the Medina, the old buildings and tannery (creation of leather from skin). The alleys in the median are really narrow and full of interesting life.
Casablanca is the largest city as well as the economic center of Morocco, but not the capital (Rabat). Casablanca is also home to the Hassan II Mosque, one of the largest in the world.
Marrakech is the most important of the four former imperial cities of Morocco. The markets, the food (more than couscous!), the colors and the sand buildings makes this place well-worth the visit.
Rabat is the capital of Morocco and one of the Imperial cities with Medina.
Ait Benhaddou is a "fortified village" where only few families live. The earthen clay structure is used for many movies (like Gladiator & Game of Thrones).
Essaouira is a very old coastal city and trading post with Medina and castles.
Merzouga is a small village near the Sahara Desert, known as a center to for local activities. Erg Chebbi, high sand-dunes, are at walkable distance.