I suspect Venice don't get CKs, not many factions do since they are top-end cavalry rather than standard units available to most factions like they were in MTW. Many factions get something similar as their top cavalry such as Famiglia Ducale or English Knights; I suppose the Venetian equivalent would be Broken Lances (the actual top-end Venetian cavalry are Stradiots, which are elite light cavalry).

As for cannon towers, they are brutally effective at long range but do suffer from a minimum range, which makes the useless once the enemy are at the gates and on the walls (which in my experience is where the real business of a siege assault takes place). Perhaps this is what is causing them to stop firing? Personally I actually prefer ballistas for out-and-out siege defense since they will keep firing in the killing zones around the gate and on the walls, but cannon towers are excellent for sally battles since the enemy will not approach within the minimum range; instead they will form up in the range of the towers and get slaughtered expecting you to sally out and attack them. This is something of an exploit though.