I've just been reading Richard Goy's book on Venice and it explains why the street system is so tortuous.
Venice didn't develop as a single contiguous city, but as a collection of islands each forming its own parish. If you'd been there around the year 1000 you wouldn't have seen a city, as you do now, with canals running through it - you would have seen mudbanks with scattered parish-islands.
Now each parish-island has quite a regular street system, usually with a main street running up to the church, and a campo in front of the church. But where these islands join up, the two different street systems have to make compromises of all kinds. So we find bridges at odd angles across the canal; dead ends; dog-legs and weird corners. Big streets running into little ones.
So I suppose I have to take back what I said about Venice being the medieval city par excellence. Perhaps it is - but it's not a paradigmatic medieval city. It's a very unusual one.