Pisa is one of the major cities in the Tuscan region. Pisa laid by the sea until the 15th century, by which time accumulated silt deposited by the Arno River had completely cut the city off from the receding shoreline. Pisa became a busy centre of woolen manufacturing late in the 13th century and remained the chief port of Tuscany. Pisan prosperity was reflected in the characteristic casatorre, a tall inhabited tower usually built of brick and stone, and in the city’s churches, particularly the grandiose and spectacular group of cathedral, baptistery, and campanile (the world’s most famous leaning tower) which constitute Piazza dei Miracoli. Pisa is now a quiet provincial university town that is renowned for its art and architectural treasures.