Most of the buildings rest on foundations of layers of wood planks, rafts of some description. On many, the attempt was simply to float it on the soft and shifting base. Many buildings were built with lighter weight materials to slow the settling, even plastered wood exteriors.

You'll see more than one tower,...leaning in and around Venice, as you must have. . It's not rare for various walls in the same building to a have foot difference in level. They knew the settling was ongoing so many of the builders didn't attach the interior walls to the exterior walls.

And then as you mention, they have to deal with the canal water wakes of salt water eating the masonry, and it's rising.

It's quite an amazing multi century ongoing restoration.

http://www.venicethefuture.com/schede/uk/338?aliusid=338