well, the Gateway Arch pretty much dominates in st louis. as the tallest monument in the united states... i suppose it should.
when completed in 1874, the Eads Bridge was the longest arch bridge in the world. it was also the first bridge to be built using cantilever support methods exclusively, and one of the first to make use of pneumatic caissons. the Eads Bridge caissons, still among the deepest ever sunk, were responsible for one of the first major outbreaks of "caisson disease" (also known as "the bends" or decompression sickness). fifteen workers died, two other workers were permanently disabled, and 77 were severely afflicted.
the Old Courthouse was Missouri's tallest habitable building from 1864 to 1894 and now is part of the Jefferson National Expansion Memorial.
the Basilica of Saint Louis, known as the Old Cathedral, was consecrated on October 26, 1834. It was the first cathedral west of the Mississippi River and until 1845 the only parish church in the city of St. Louis
Forest Park opened in 1876 and is the former site of the Louisiana Purchase Exposition of 1904, (better known as "The World's Fair"). its one of the largest urban landscape parks created during the later 19th century, following the example of Central Park in New York City. at 1,293 acres (5.2 km²), Forest Park is over 50% larger than New York's Central Park (843 acres or 3.41 km²). the park also contains the art museum, the jewel house, the zoo, a skating rink, a golf course, a tennis center and the Muny (outdoor Opera Theatre).
Art Museum in Forest Park, built as the Palace of the Fine Arts for the 1904 World's Fair. its considered one of the principal U.S. art museums, visited by up to a half million people every year. Admission is free
Founded in 1859, the Missouri Botanical Garden is one of the oldest botanical institutions in the United States and a National Historic Landmark.
there are far too many interesting old churches to show. you can see some of them here:
http://www.builtstlouis.net/churches/index.html