Question: What shape was the bastion fort whose defense inspired the poem that the American national anthem was based on?

Context paragraphs:
Fort Stanwix: Fort Stanwix was a colonial fort whose construction commenced on August 26, 1758, under the direction of British General John Stanwix, at the location of present-day Rome, New York, but was not completed until about 1762.  The star fort was built to guard a portage known as the "Oneida Carrying Place" during the French and Indian War.  Fort Stanwix National Monument, a reconstructed structure built by the National Park Service, now occupies the site.

Fort McHenry: Fort McHenry, in Baltimore, Maryland, is a historical American coastal pentagonal bastion fort best known for its role in the War of 1812, when it successfully defended Baltimore Harbor from an attack by the British navy from the Chesapeake Bay September 13–14, 1814.  It was first built in 1798 and was used continuously by U.S. armed forces through World War I and by the Coast Guard in World War II.  It was designated a national park in 1925, and in 1939 was redesignated a "National Monument and Historic Shrine".

National Anthem Project: The National Anthem Project was a public awareness campaign launched in 2005 as a major initiative of .  At the time of its launch, the National Anthem Project website declared "MENC is sponsoring The National Anthem Project to revive America's patriotism by educating Americans about the importance of The Star-Spangled Banner-both the flag and the song."  Sponsored by MENC with major support from the Jeep brand, and other sponsors such as NAMM, Bank of America, and the Gibson Foundation, this campaign, which later used the slogan "to restore America's voice through music education" was MENC's most ambitious project to date.  A former First Lady, Laura Bush served as honorary chairperson, with country music's The Oak Ridge Boys as the official musical ambassadors.  The stated purpose of the project was originally "to revive America's patriotism," but this was later modified to suggest that its purpose was merely to encourage more singing of the national anthem, or to bring more public attention to the role of music in American schools.

Nordmannen: Nordmannen ("The Norwegian") is a Norwegian poem written by Ivar Aasen.  The poem is better known in Norway as "Mellom bakkar og berg" (Literally "Between hills and mountains"), and was published for the first time in the collection of poems called "Symra" in 1863.  Some parts of this poem are widely sung in Norway, and the two first stanzas were engraved on all Norwegian driving licences from 1998 to 30 June 2007.  It is generally considered the third most important national song in Western Norway after the national anthem itself Ja, vi elsker and .  It was in 2005 chosen as the national anthem for Western Norway by the founders of the Republic of Gulating.

Fort Independence (Massachusetts): Fort Independence is a granite bastion fort that provided harbor defenses for Boston, Massachusetts.  Located on Castle Island, Fort Independence is one of the oldest continuously fortified sites of English origin in the United States.  The first primitive fortification, called "The Castle", was placed on the site in 1634 and, after two re-buildings, replaced circa 1692 with a more substantial structure known as Castle William.  Re-built after it was abandoned by the British during the American Revolution, Castle William was renamed Fort Adams and then Fort Independence.  The existing granite fort was constructed between 1833 and 1851.  Today it is preserved as a state park and fires occasional ceremonial salutes.  Fort Independence was added to the National Register of Historic Places in 1970.

Wilhelmus: Wilhelmus van Nassouwe, usually known just as the Wilhelmus (Dutch: "Het Wilhelmus" ; ] ; English translation: the "William"), is the national anthem of the Kingdom of the Netherlands.  It dates back to at least 1572, making it the oldest known national anthem in the world.  The national anthem of Japan, Kimigayo, has the oldest lyrics, dating from the 9th century.  However, a melody was added only in the late 19th century, making it a poem rather than an anthem for most of its lifespan.  Although the "Wilhelmus" was not recognised as the official national anthem until 1932, it has always been popular with parts of the Dutch population and resurfaced on several occasions in the course of Dutch history before gaining its present status.  It was also the anthem of the Netherlands Antilles from 1954 to 1964.

Arippu fort: Arippu Fort (Sinhalese: අරිප්පු බලකොටුව ; Tamil: அரிப்புக் கோட்டை , also known as Allirani fort; Tamil: அல்லிராணிக் கோட்டை ) was built by the Portuguese and was handed over to the Dutch in 1658.  The small bastion fort is located in Arippu, which is 16 km away from Mannar Island.  The fort is nearly square in shape, with two bastions.

Hymn to Liberty: The "Hymn to Liberty" or "Hymn to Freedom" (Greek: Ύμνος εις την Ελευθερίαν , "Ýmnos is tin Eleftherían" ] , also Greek: Υμνος προς την Ελευθερίαν "Ýmnos pros tin Eleftherían" ] ) is a poem written by Dionysios Solomos in 1823 that consists of 158 stanzas, which is used as the national anthem of Greece and Cyprus.  It was set to music by Nikolaos Mantzaros, and is the longest national anthem in the world by length of text.  In 1865, the first three stanzas (and later the first two) officially became the national anthem of Greece and, from 1966, also that of the Republic of Cyprus.

The Star-Spangled Banner: "The Star-Spangled Banner" is the national anthem of the United States of America.  The lyrics come from "Defence of Fort M'Henry", a poem written on September 14, 1814, by the 35-year-old lawyer and amateur poet Francis Scott Key after witnessing the bombardment of Fort McHenry by British ships of the Royal Navy in Baltimore Harbor during the Battle of Baltimore in the War of 1812.  Key was inspired by the large American flag, the Star-Spangled Banner, flying triumphantly above the fort during the American victory.

Himno Nacional Mexicano: The "Mexican National Anthem" (Spanish: "Himno Nacional Mexicano" ), also known as "Mexicans, at the cry of war" (Spanish: "Mexicanos, al grito de guerra" ), is the national anthem of the United Mexican States.  The anthem was first used in 1854, although it was not officially adopted "de jure" until 1943.  The lyrics of the national anthem, which allude to historical Mexican military victories in the heat of battle and including cries of defending the homeland, were composed by poet Francisco González Bocanegra after a Federal contest in 1853.  Later, in 1854, he asked Jaime Nunó to compose the music which now accompanies González's poem.  The anthem, consisting of ten stanzas and a chorus, effectively entered into use on September 16, 1854.

Supporting paragraphs: The Star-Spangled Banner Fort McHenry
Answer: pentagonal