It is otherwise known as Piazza dei Miracoli ("Square of Miracles"). This name was created by the Italian writer and poet Gabriele d'Annunzio who, in his novel Forse che si forse che no (1910) described the square in this way:
L’Ardea roteò nel cielo di Cristo, sul prato dei Miracoli - which means: "The Ardea rotated over the sky of Christ, over the meadow of Miracles."
Often people tend to mistake the term with Campo dei Miracoli ("Field of Miracles"). This one is a fictional magical field in the book Pinocchio, where a gold coin seed will grow a money tree.
In 1987 the whole square was declared a UNESCO World Heritage Site."
"The Leaning Tower of Pisa is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa. It is situated behind the Cathedral and it is the third structure by time in Pisa's Piazza del Duomo (Cathedral Square).
Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 due to a poorly laid foundation and loose substrate that has allowed the foundation to shift direction. The tower presently leans to the southwest." - in: wikipedia.