Bobastro was the hilltop redoubt of 9th-century rebel Omar ibn Hafsun, who led a prolonged revolt against Córdoban rule. At one stage he controlled territory from Cartagena to the Strait of Gibraltar. It's thought that he converted from Islam to Christianity (thus becoming what was known as a Mozarab) before his death in 917 and was buried here. When Córdoba conquered Bobastro in 927, the poor chap's remains were taken for grisly posthumous crucifixion outside Córdoba's Mezquita.
From El Chorro village, drive up the far (western) side of the valley and after 3km take the signed Bobastro turnoff on your left. The ticket booth is another 3km up the hill; from here, follow a 500m path to the remains of a remarkable little Mozarabic church cut from the rock, the shape so blurred by time that it appears to have been sculpted by the wind alone.
Alternatively, you can hike 4km to Bobastro from El Chorro village. Take the road downhill from the station, cross the dam and turn left after 400m at the GR7 trail signpost. The first 2km are steep uphill, and then it's flat and slightly downhill. The views of El Chorro gorge are marvellous.
At the top of the hill, 2.5km up the road from the ticket booth and with unbelievable views, are faint traces of Ibn Hafsun's rectangular alcázar (Muslim-era fortress).