The archaeological excavation undertaken at Quinta de Crestelos (Merinhos, Mogadouro, Portugal), revealed a large occupation site dated between Late Iron Age and Early Roman periods. On the ridge top a small fortified site with elaborated defensive structures, consisting of large stone walls and ditches, showed several different phases of construction and large modifications. The different phases of the site evidenced the ‘Romanization’ Process of this area and the large changes occurring in the Late Iron Age sites, with defensive structures, such as the original ditches, reused as functional and production areas. Paleomagnetism studies and analysis of the combustion structures of the later phases of the site intended to clarify nature, main components and changes during the time when they were used.