The kasbah of Agadir must be the worst sight of the country. Except from parts of the walls and the gate, there is nothing to attract anyone's interest. Yet, there are numerous bus loads coming up here, and watching all the tourists walk around the area wondering "what the heck is this good for?" would be your best entertainment.
There is only one great thing to find up here: the great view over Agadir. And this is is definately worth the 20 minute ascend by foot, or 3 minutes in a car.