Tower of l'Abella Monument · la Riera de Gaià, Tarragonès, Tarragona Circular stone tower probably raised around the year 1000, when the Gaià marked the frontier with al-Andalus, to watch over the road at the entrance to the village. About seven meters of elevation survive, with stretches of opus spicatum and remains of the vault that covered it; it is half-ruined and covered in vegetation atop a small hill next to the road. Photo: lluis_tgn (CC BY-SA 3.0) What to see and do in la Riera de Gaià