This is another tower nearby and I don't know it's name. But like many other towers built back in the old days, the unstable ground tends to compress on one side or the other. The leaning tower of Pisa is not so unusual as I once thought.
Here is a tower on the island of Burano, which is out in the Venetian Lagoon. It too has a decided lean. It seemed like there were plenty of towers in the Venice area and about half were leaning one way or another. The towers were built as lighthouses and places to observe shipping, friendly, enemy and commercial.
And here is something seldom seen. Saint Mark's with hardly a person in it. We got up fairly early by Venetian standards and were the first persons on the elevator up the Campanile. The nine o'clock bells were still ringing when we got up top nearly deafening us as we stepped out of the elevator. You can see the shadow of the tower stretching across Saint Mark's square. This shadow makes the tower look like it has a lean, but it is plumb.
The striking thing about this photo from the top of the Campanile is that it displays how vulnerable the islands in the lagoon are to high water. From this vantage point it looks like a decent high tide would swamp this little place, but you must remember that high tides in the Mediterranean are not as high as in the Atlantic and Pacific. You can also see a fraction of the water traffic on the end of the Grand Canal near San Marco. Barges, vaporettos, gondolas and private craft performing a water dance. And you decide. Does that tower in the distance have a lean as well?