The castle needs a lot of materials delivered by ship and the cog was just the ship to deliver them.
Never heard of a “cog”? Neither had I. It turns out that it was the dominant type of merchant ship in Northern Europe from the 13-17th centuries and was quite revolutionary during a time of a huge increase in maritime trading.
The cog was fast and could carry a large cargo on its flat floor. It was broad in beam (width), had a keel (the backbone that gave it great strength) and was built in the clinker (lapstrake) style. This meant that the hull outer boards overlapped each other rather than being edge to edge.
The “castle” or raised portion was in the rear and the forecastle was above the bow. The same style of ship was used as a warship and was fitted out accordingly.