Castle was born in Essex, Vermont. The family moved to Toronto in 1815, where his father, Jonathan Castle, was engaged as an architect "to superintend the construction of the first Parliament buildings there." In 1827, the family settled in Cleveland.
Jonathan and William Castle opened Cleveland"s first lumberyard. He moved back to Ontario and then to Ohio City in 1839. In 1853, he was elected the city"s mayor and helped in writing the 1854 agreement to merge Cleveland and Ohio City.
In 1855, he became mayor of Cleveland.
On July 21, 1862, Castle, as chairman of the District Military Committee, Cleveland, sent a letter to Governor David Tod, enclosing a copy of a resolution recommending that the appointment of company officers for the 103rd Regiment, Ohio Volunteer Infantry. In December 1836, Castle married Mission Mary Derby, who died in Canada the following year.
Mayor Castle was a delegate to Republican National Convention from Ohio in 1868. There is a large historic marker on the eastern wall of the church in his honor.
Mayor Castle was interred in the Lake View Cemetery, in Cleveland, Ohio.
He entered politics and quickly rose to prominence as a member of the Ohio City Common Council.
