 In England, where the principle tower is usually over the crossing, the west front may be framed by large turrets.In Italy, with the exception of Milan Cathedral, the form of a Gothic west front is less strongly architectonic and sculptural than in other parts of Europe.The underlying structure may be brick, rather than stone, overlaid with a veneer of polychrome marble, and ornamented with marble sculpture and coloured mosaics as at Siena and Orvieto Cathedrals.