The ruined eighteenth-century palace of Sultan Sidi Mohammed sits on a low hill across the Oued Ksob, just outside the Berber village of Diabat south of town. The view back across the river to Essaouira and the Iles Purpuraires is the reward; the half-buried walls and empty arched chambers are pleasant to wander but historically thin. Combine the walk with the so-called Castle Made of Sand on the beach below, the ruined fort that supposedly inspired Jimi Hendrix's song.
To reach it: walk south down the beach to the ruin on the sand, ford the river at low tide (or detour to the bridge near Diabat), then climb the trails on the far bank. A camel ride down the beach is the lazy option.