Which is the only U.S. city that has a royal palace?

'Iolani Palace is the only royal palace on US soil.

The ʻIolani Palace was the royal residence of the rulers of the Kingdom of Hawaii beginning with Kamehameha III under the Kamehameha Dynasty (1845) and ending with Queen Liliʻuokalani (1893) under the Kalākaua Dynasty, founded by her brother, King David Kalākaua. It is located in the capitol district of downtown Honolulu in the U.S. state of Hawaii. It is now a National Historic Landmark listed on the National Register of Historic Places. After the monarchy was overthrown in 1893, the building was used as the capitol building for the Provisional Government, Republic, Territory, and State of Hawaiʻi until 1969. The palace was restored and opened to the public as a museum in 1978.