Sengan yagura, the story of a name

Along the outer moat surrounding the Osaka Castle there are multiple towers, called yagura. The one on the right side of the photo, built by the Tokugawa shogunate in 1622, has the trivial name of Roku-ban Yagura, meaning "Turret number 6".

But the tower on the left side of the photo has a much more interesting name, Sengan Yagura, evoking a story from the Sengoku period: before the construction of the Osaka Castle, this place was occupied by Ishiyama Hongan-ji, a fortress-temple built by the Ikkō-ikki warriors.

In 1576, when Oda Nobunaga tried to conquer the fortress, he received an injury from an arrow shot from that turret. Angered, he said then that he will give sen-gan-mon to the one who will conquer the turret.

Sen means "thousand" and the kanmon was an old Japanese monetary unit, referring to 1,000 mon... Another version is that kan was referring to the old measure for mass, equivalent to 3.75 kilograms (thus Oda's offer was 3 and 3/4 tons of mon).
Anyway, that was a huge amount of money... So, 44 years later, when the Osaka Castle was built on the same place, the tower was kept with this unusual name...