According to the site's current architect, Pedro Salmeron Escobar, the Alhambra evolved organically over a period of several centuries from the ancient hilltop fortress defined by a narrow promontory carved by the river Darro and overlooking the Vega or Plain of Granada as it descends from the Sierra Nevada. The red earth from which the fortress is constructed is a granular aggregate held together by a medium of red clay which gives the resulting layered brick- and stone- reinforced construction (tapial calicastrado) its characteristic hue and is at the root of the name of 'the Red Hill'.