Titan Eric's solution is perfect, this is just an advice:
you can start from Monteriggioni, of course, then fast travel to Forlì (south), now steal an horse and go to Tuscain (in this way you'll cross all the mountains - Appennini - quite boring, I know), from Tuscain San Giminiano, you can reach Florence.
Now fast traver to Venice.
In this way you'll get the achievement with no bad encourter: Venice is the most dangerous city to wear the Auditore Cape, but in this way you'll get it in the loading screen, so you can drop off the cape immediately.