Black Powder Cannon is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at Mongol Shipyard in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Head to Mongol Shipyard in Izuhara and search the area for the item. Mongol Shipyard is a Mongol-held territory, so expect to fight or sneak past guards before you can search it.
Also at Mongol Shipyard: the record Conversations with the Khan 2 and 4 Sashimono Banners you get for liberating it.
Conversations with the Khan 2
Conversations with the Khan 2 is one of the Conversations with the Khan records, left by locals during the Mongol invasion. You find it at Mongol Shipyard in Izuhara, and it counts toward the Avid Reader trophy.
Aaruul
Aaruul is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at a camp south of Izuhara Clearing in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Adak Tree
Adak Tree is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at Yoichi's Crossroads in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.