He is credited with winning control of the Hawaiian Commercial & Sugar Company from Claus Spreckels in 1898, which he sold to Alexander & Baldwin for a large share of their stock.

Later in 1897, he served as secretary of the delegation from the Republic of Hawaii sent to Washington, D.C., to lobby for annexation at the request of Lorrin Andrews Thurston.

The last failed effort was organized by Peter Demens who offered to bring all Spiritual Christians from Russia, who were immigrating to Canada, to Hawaii, which cost Castle about $30,000.

Their son Harold Kainalu Long Castle (1886–1967) expanded the business and donated land for several educational institutions.

Two days later Benjamin Dillingham died, who married a distant cousin, and partnered in building railroads to the Castle plantations.