The Arsenale rises in the Sestriere Castello, it was founded in the 12th century and by the 16th it had become the greatest naval shipyard in the world (able to build a whole galley in 24 hours). It was like a city within a city.
Today the site is almost unused but you can admire its impressive gateway, twin towers and guardian lions. You can also take the vaporetto number 23 or 52 that will take you in the heart of the Arsenale Vecchio and you can see the docks and the warehouses inside the walls.
Not far from the Arsenale, you can visit also the Naval Museum.