The Alcazaba is the oldest part of the Alhambra. It was built in the 13th century on the remains of the earlier fortifications dating back from Carthaginians and Romans. 

The Entrance of the Alcazaba

Its exceptional position made it a perfect quarter to house the soldiers and guards of the rulers of the Alhambra. You can still see the foundations of the houses and stores.
Courtyard of the Alcazaba seen from the top of Torre de la Vela

The Alcazaba offers a great panorama over the district called the Albaycín, on the other bank of the river Darro

Albaycin seen from the Alcazaba round tower
The Alcazaba has a triangular shape and at the tip of it stands the famous Torre de la Vela :

Torre de la Vela, facing the modern Granada