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The Alton Gas & Electric Power House is a former hydroelectric power plant at 700 W. Broadway in Alton, Illinois.

The Christian Hill Historic District is a historic district and neighborhood in Alton, Illinois. The district is located west of Alton's central business district on a bluff overlooking the Mississippi River. A primarily residential area, the district includes 274 buildings, of which 266 are contributing buildings to its historic character. Development in the district began in the 1830s; in addition to residential development, the Illinois State Prison was constructed in the neighborhood in 1833. While Alton's economy declined during the Panic of 1837, development in the district continued through the early 20th century, and Christian Hill became a desirable neighborhood for Alton residents. The hill acquired its name from, Saints Peter and Paul Church, a Catholic church built in the 1850s which was the cathedral of the Diocese of Alton until the diocese moved to Springfield.The historic district was added to the National Register of Historic Places on May 22, 1978.

The Post House is a historic house located at 1516 State St. in Alton, Illinois, United States. William Post, a steamboat captain who later became mayor of Alton, built the house in 1837–38. The brick and limestone house is designed in the Greek Revival style. The house's front facade features four Doric columns topped by an entablature and a pedimented gable end. The front porch of the house wraps around both sides, each of which has an additional column and a pilaster. The cornice and front pediment are both dentillated. James Patterson, owner of the Illinois Iron Works, purchased the house in 1854; Patterson may have added the iron porch railing.The house was added to the National Register of Historic Places on May 28, 1980.

The Alton Chapter House is a historic building located at 509 Beacon St. in Alton, Illinois. The building was constructed between 1909 and 1910 as a meetinghouse for Alton's chapter of the American Woman's League. The American Woman's League was a political and social organization founded by magazine publisher Edward Gardner Lewis in 1908. The organization was created to promote feminist causes, particularly the women's suffrage movement; Lewis also intended for the organization to promote and sell his women's magazines. Lewis commissioned the St. Louis architectural firm of Helfensteller, Hirsch & Watson to design five classes of buildings which the League would use as meetinghouses. The Alton Chapter House is an example of a Class II building, which was designed for clubs with 60 to 100 members. The building was designed in the Prairie School style and cost $2,500. After the club disbanded, the building was converted to a private home.The building was added to the National Register of Historic Places on November 28, 1980. It is one of nine American Woman's League chapter houses on the National Register in Illinois.

The Alton Military Prison was a prison located in Alton, Illinois, built in 1833 as the first state penitentiary in Illinois and closed in 1857. During the American Civil War, the prison was reopened in 1862 to accommodate the growing population of Confederate prisoners of war and ceased to be prison at the end of the war in 1865. The prison building was demolished not long after the Civil War. All that remains of the former prison site is a section of ruin wall that is maintained by the State of Illinois as an historic site. The prison site is included in the U.S. National Register of Historic Places.

The Hotel Stratford is a historic hotel located at 229 Market St. in Alton, Illinois. The hotel, originally known as the Illini Hotel, opened in 1909. St. Louis architectural firm Barnett, Haynes & Barnett designed the hotel in the Classical Revival style. The five-story brick building features brick quoins, limestone bands and window sills, terra cotta ornamentation, and a cantilevered cornice. The hotel opened a rooftop garden in 1910 and a reception room in 1912, and it became one of many hotels which drew conventions to Alton; a contemporary newspaper account described the building as part of the "greatest improvement in property in the city of Alton". In 1925, new owner E. J. Lockyer renamed the hotel to its current name. The hotel is the only hotel built in Alton before 1950 that is still in operation.The hotel was added to the National Register of Historic Places on January 27, 2000.

The Guertler House is a historic house located at 101 Blair St. in Alton, Illinois. Stonemason Ignaz Bruch built the house in 1854. Bruch, who immigrated to the United States from Germany in 1846, was a prominent Illinois stonemason who constructed buildings in cities ranging from Chicago to St. Louis, Missouri. The Guertler House is a Federal building with three bays. The front door and windows have a pointed arch design, while the second story of the house's west side has a single Gothic arch window.The house was added to the National Register of Historic Places on July 30, 1974.

Marquette Catholic High School is a private, Catholic high school in Alton, Illinois. It is located in the Diocese of Springfield in Illinois.

McPike Mansion, or Mount Lookout, is a mansion in Alton, which is part of the Metro-East region of the Greater St. Louis metropolitan area in the U.S. state of Illinois. Built in 1869 by Henry Guest McPike (1825–1910), it is situated on Alby Street on a site of 15 acres (61,000 m2), one of the highest points in Alton, which was called Mount Lookout.

The Alton Bridge was a railroad bridge that carried the Chicago, Burlington, and Quincy (later Burlington Northern) across the Mississippi River between West Alton, Missouri, and Alton, Illinois. It was located 100 yards (91 m) upriver from, and parallel to, the Old Clark Bridge. The bridge was built between 1892 and 1894 and was removed shortly after the line was abandoned in 1988. With a total length of 2,060 feet (630 m), it consisted of eight through-truss segments—six Pratt trusses, one Pennsylvania truss, and one pivot-swinging truss.

Lock and Dam No. 26 was a lock and dam located near Alton, Illinois on the Upper Mississippi River around river mile 202.5. Opened in 1938, its largest lock was 600 feet long. It was demolished in 1990 and replaced by the Melvin Price Locks and Dam, which is also known as Lock and Dam number 26.

Lyman Trumbull House is a house significant for its association with former U.S. Senator from Illinois Lyman Trumbull. The house is located in the historic Middletown neighborhood in Alton, Illinois. Senator Trumbull was best known for being a co-author of the Thirteenth Amendment to the United States Constitution. The house was built around 1849, and was declared a National Historic Landmark in 1975. Senator Trumbull lived in this house from 1849 to 1863, according to the documentation provided in the National Historic Landmark application. The house is a 1+1⁄2-story red brick, gable-roofed residence with limestone foundation. It was originally rectangular-shaped, but late in the 19th century an addition was built on the rear of the house, transforming it into an "L" shaped residence. There are three gabled dormers protruding from the front roof, one on the rear of the original house, and one on the northern elevation of the roof on the addition. Adorning the front of the house is a centrally-located one-bay entrance porch supported by two fluted pilasters, all made of wood. Turned balusters flank the porch and the several wooden steps that lead to a brick walkway surrounding the dwelling. An entrance to the basement is located underneath the porch. The chief front entrance to the Trumbull House is a single door with side lights and semi-elliptical fanlight. On the south side of the house is a second basement entrance, and it is sheltered by a pedimented portico supported by two Doric columns.