There are a couple of component parts to the Medina that warrant a visit in their own right. 
The Ribat, an impressive fort dating from the 8th century that is surprisingly complete and perfectly maintained. The site can be visited including the tower.
The Great Mosque of Sousse (picture) dates from the 9th century and is impressive in it's size and layout. It doesn't fit the stereotypical image of a mosque, a domed building with a slender minaret, it is squat and rectangular and has a defensive nature to it being enclosed behind formidable walls with round towers on the corners surrounding a tranquil and elegant inner courtyard.
And the Medina itself with it's own defensive walls and tower and magnificent city gates make it one of the most interesting places in Tunisia.