Atakebune were the largest class of purpose built warships that were used by the Japanese clans during the Sengoku period, or the Warring States period. These ships ranged from around 30 to 50 meters in length, were equipped with a large, box-like structure. Inside were the oarsmen, foot soldiers and samurai, protected by the wooden walls. The structure had two or three levels, with the top level being the roof of the structure. Firing and viewing ports were cut out and may have been closable with a hinged cover.

Atakebune model at the Verkehr Museum in Shizuoka.
In addition to a single-bank of sculling oars, the ship carried a large square sail hung from a single mast, usually mounted near the center of the ship. In bad weather, or when otherwise not in use, the mast could be un-stepped and lowered across the top of the ship. Usually, the ships were equipped with three sets of supports that the masts laid across.
Some ships carried a heavily constructe deck cabin that sat of the roof level of the ship. Some unusually large atakebune, referred to as an o-atakebune, carried castle-like structure atop.

Image courtesy of the University of Tokyo General Library – Atakemaru ship illustration / image edited