The trust said its historical uses included communal occupation, farming, feasting, conflict, and burial.
It is the first hill fort acquired by the trust in Dorset for 30 years and joins its six other sites in the county, including Hod Hill, Lamberts Castle, Badbury Rings and Pilsdon Pen.
The National Trust said it decided to buy the site to secure its future, and ensure maintenance and access for the public was maintained.
The chalk grassland is home to at least five species of orchids, while 28 species of butterfly have been recorded at the site over the years.
From its summit visitors can see across three counties: Dorset, Somerset, and Wiltshire.
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