Arnoia waterfalls in A Merca Nature · A Merca, Terra de Celanova, Ourense Image of A Merca, not of the place itself. The Arnoia drops over a series of waterfalls on this stretch before continuing on its way towards the Miño; along its bank four Iron Age castros are lined up: Peneda da Lebre, Outeiro de San Marcos, Rodela and Monte do Castro. Photo: LIMIAO (CC BY-SA 4.0) What to see and do in A Merca