The Cathedral, begun in 1131, in a style of Norman architecture which would be more accurately called Sicilian Romanesque. The exterior is well preserved, and is largely decorated with interlacing pointed arches; the windows also are pointed. On each side of the façade is a massive tower of four storeys. The round-headed Norman portal is worthy of note. A semi-circular apse is set into the east end wall. Its strengthening counterforts that work like buttresses, are shaped as paired columns to lighten their aspect. The groined vaulting of the roof is visible in the choir and the right transept, while the rest of the church has a wooden roof. Fine cloisters, coeval with the cathedral, adjoin it.