While the maindeck lay completely open to the Deep, protected only by the §-field which, in addition to making the act of starsailing possible, allowed mast, spars, and sails to be worked by crewbeings attired for a "shirtsleeve” environment, there were lower decks meant for cargo, passengers, and a formidable array of projectibles bearing much the same relationship to thrustibles that cannon did to pistols.
Source: wiktionary