Which evidence in the passage best supports the inference that there is mystery behind the design of the Leaning Tower?

excerpt from The Innocents Abroad
by Mark Twain

At Pisa we climbed up to the top of the strangest structure the world has any knowledge of—the Leaning Tower. As everyone knows, it is in the neighborhood of one hundred and eighty feet high—and I beg to observe that one hundred and eighty feet reach to about the height of four ordinary three-story buildings piled one on top of the other, and is a very considerable altitude for a tower of uniform thickness to aspire to, even when it stands upright—yet this one leans more than thirteen feet out of the perpendicular. It is seven hundred years old, but neither history or tradition say whether it was built as it is, purposely, or whether one of its sides has settled. There is no record that it ever stood straight up. It is built of marble. It is an airy and a beautiful structure, and each of its eight stories is encircled by fluted columns, some of marble and some of granite, with Corinthian capitals that were handsome when they were new. It is a bell tower, and in its top hangs a chime of ancient bells. The winding staircase within is dark, but one always knows which side of the tower he is on because of his naturally gravitating from one side to the other of the staircase with the rise or dip of the tower. Some of the stone steps are foot-worn only on one end; others only on the other end; others only in the middle. To look down into the tower from the top is like looking down into a tilted well. A rope that hangs from the center of the top touches the wall before it reaches the bottom. Standing on the summit, one does not feel altogether comfortable when he looks down from the high side; but to crawl on your breast to the verge on the lower side and try to stretch your neck out far enough to see the base of the tower, makes your flesh creep, and convinces you for a single moment in spite of all your philosophy, that the building is falling. . . .