This is the worst transportation…
This is the worst transportation company in Europe, even worse than in some third world countries. During the summer, the peak tourists season, the buses are completely undependable, usually late and often not showing up at all. It is especially bad on weekends when you rarely see a bus much less get the opportunity to take one. So unless you have infinite patience you are left to walk around this disgusting city, through all its filthy streets and sidewalks covered in tough on the feet ancient stones.
Above all, do not buy too many tickets in advance as you most likely will be wasting your money because chances are good there won't be a bus to use them on. That means, unfortunately, that everytime you are willing to roll the dice and try to find a bus you will first have to hunt down a tobacco store or newsstand to buy a ticket. You can't pay on board. The driver won't take your money and they don't have a credit card tap-and-go that works.
The funny part is that when you do actually get on a bus you'll see that 90% of the locals don't even punch tickets. So you could do likewise and not even bother to buy tickets, but then if an inspector did miraculously appear you as a tourist would probably be fined heavily while the locals would get off scott free.