Towns of roman origin usually show some traces of their ancient past. The Roman walls often formed the basis of the medieval circuit,as at Trier and London ,and Roman buildings were frequently converted to other uses-at Arles in Provence the amphitheatre was turned into a fortification to protect the little town that had grown up within it-or their materials reused .The Roman grid of streets influenced the medieval lay-out ,and at Verona the marketplace was on the site of the old forum; equally,medieval streets might cut across the Roman network or preserve the outlines of a ruin, as at Lucca, where the amphitheatre formed the marketplace . At merida in Spain the Roman plan, bridge over the river and several buildings survived Muslim occupation and Christian reconquest.