Explain to me why this gets to be its own country?

Took a road trip this weekend to the Most Serene Republic of San Marino, which is a tiny little landlocked city in Italy, a little south of Venice on the east coast and about six miles from the water. It's a classic medieval city, built on a giant hill, with a palace and giant watchtowers and keeps and battlements and a billion shops selling crossbows and the whole nine yards. It was actually really interesting. There are no border formalities, but it is indeed it's own country, having maintained independence during the unification of Italy in 1880-something (you'd think I'd know that by now). One of the nice things is the lack of sales tax, meaning lots of great stuff like inexpensive leather and alcohol. But really, it was just great to walk around and explore it--the old city is very small, about the size of Montelpulciano (mentioned in a post September of this year, but I'm too lazy to link it right now). Highly recommended as a side trip if you happen to be in central Italy anytime soon.