You can catch a really cheap flight from Sevilla to Fez, and Fez is 1000x better than Tangier.  Tangier is basically the Tijuana of Morocco. Small, very touristy and not near the history as other cities in Morocco.  Fez is by far the best city to see in Morocco if you can only see one.  Incredible history, the Medina is much larger than Marakesh (which is also very touristy), and is very easily accesible via flight from Spain.

Marrakech has some cool stuff----like the snake charmers in the Jemaa-el-Fnaa, but is relatively small and the souks are much smaller than those in Fez (plus they don't have some of the really cool things like the tannery in Fez).  If you are going to go out to the Saraha, Marrakech is a good jumping point (although its still an 8-hour drive to get there).

Everyone has a very romantic image of Casablanca because of the movie, but really isn't that interesting.  Just a big, third world city, although it does have an amazing mosque that is one of the few that non-Muslims can enter.

Chefchaouen (the blue city) is incredible, but is somewhat remote and hard to access.