Tower of l'Alzina Monument · la Riera de Gaià, Tarragonès, Tarragona Image of la Riera de Gaià, not of the place itself. Of this circular defence tower, situated on a hillock reached from el Catllar via the camí de Coll Canut, only the plan excavated in the rock remains: 4.15 metres in diameter and walls 105 centimetres thick, of stone bonded with lime mortar and river sand. It was preserved up to about two metres in height until in 1987 a digger destroyed the remains, for reasons unknown. There are no objective dating elements, but its visual relationship with other towers and castles on the frontier line of the Baix Gaià, such as the torre de la Vella and the castle of Catllar, places its construction in the 11th century. Photo: Darth vader 92 (CC BY-SA 4.0) What to see and do in la Riera de Gaià