The lid is half open, and features an observation window and a ventilation opening, shown half-opened. It opens by pulling two chains that run from the top of the lid and through the hull, and closes by pulling chains running from below the lid and through the gun port. The lid itself has two layers: an inner layer made of vertical planks, providing structural integrity and watertightning, and an outer layer made of horizontal planks that integrates into the outer hull. A short roof above the gun port offers some protection from the elements, and prevents the lid from jamming against the hull.
A gunport is an opening in the side of the hull of a ship, above the waterline, which allows the muzzle of artillery pieces mounted on the gun deck to fire outside. The origin of this technology is not precisely known, but can be traced back to the late 15th century, with the appearance of artillery in naval warfare. Ships featuring gunports were said to be pierced, since the ports were cut through the hull after the construction.