This is the entry into the fish market. The words "PUBBLICO MERCATO" - Public Market- are written above the central arch.
This plaque provides some historical information about the site. It appears that the St. Francis Monastery was built here in 1220, was expanded in 1300 and then, for some reason, demolished in 1880. I'm not sure how old the current building is exactly, but obviously no older than 1880.