What is a question about this article? If the question is unanswerable, say "unanswerable".

In the first decades of the 19th century, Federal architecture and Greek Revival architecture were dominated by Philadelphia architects such as Benjamin Latrobe, William Strickland, John Haviland, John Notman, Thomas U. Walter, and Samuel Sloan. Frank Furness is considered Philadelphia's greatest architect of the second half of the 19th century, but his contemporaries included John McArthur, Jr., Addison Hutton, Wilson Eyre, the Wilson Brothers, and Horace Trumbauer. In 1871, construction began on the Second Empire-style Philadelphia City Hall. The Philadelphia Historical Commission was created in 1955 to preserve the cultural and architectural history of the city. The commission maintains the Philadelphia Register of Historic Places, adding historic buildings, structures, sites, objects and districts as it sees fit.

When did building begin on the Philadelphia CIty Hall?
1871