These are the gates to the Arsenale, the famed shipyards which were the source of much of the Venetian Republic's wealth. There used to be a small Renaissance church to the right of these gates and thanks to a painting by Canaletto, we can see what it looked like.
Madonna dell 'Arsenale was built in the 16th century by an unknown architect who modeled it on a Greek temple. Looks like a sweet little place. It was demolished in the early 19th century.
In the painting, you can see a relief of the lion of San Marco over the door to the church and maybe some mosaics underneath that.
This little church housed an altar for a revered 14th century wooden crucifix that was carried on a Venetian ship during the 1571 Battle of Lepanto. This crucifix is now in the nearby church of San Martino.