Castello is the biggest, least touristy sestiere — laundry-strung working streets, the green Giardini, and the monumental walled Arsenale, the medieval shipyard that built the Republic's fleet and once employed thousands. Walk east of the Riva and the crowds vanish. The catch: the Arsenale interior is mostly closed except during the Biennale, so off-season you're admiring gates and walls and absorbing atmosphere rather than touring a site.