 One of the most famous architects in the United States at the turn of the 20th century, his popularity rose after being selected as the architect of what is now Grant's Tomb, another "reconstruction" of the Mausoleum of Halicarnassus (Greek Ionia).Another of Duncan's most famous works is the Soldiers' and Sailors' Memorial Arch in Grand Army Plaza in Brooklyn New York, often referred to as Brooklyn's version of the Arc de Triomphe.Duncan also contributed townhouses on Manhattan's Upper East side.