Short answers, pulled from the story.
The Nippon Budokan was built specifically to host judo at the 1964 Summer Olympics in Tokyo, the first time judo had been included as an official Olympic sport. It was completed on the 15th of September 1964 and opened with an Olympic judo competition from the 20th to the 23rd of October that year.
The Beatles played five concerts at the Nippon Budokan between the 30th of June and the 2nd of July 1966, becoming the first rock group to perform there. Each concert lasted 30 minutes. Their appearances drew opposition from people who felt a Western pop group would defile the martial arts arena.
The Nippon Budokan holds 14,471 people in total, made up of 2,946 arena seats, 3,199 first floor seats, 7,846 second floor seats, and 480 standee places. The octagonal building rises 42 meters high.
Eikichi Yazawa holds the record for the most music concerts at the Nippon Budokan, having performed there 142 times as of the 19th of December 2017.
Cheap Trick recorded Cheap Trick at Budokan there in 1978, and Bob Dylan recorded Bob Dylan at Budokan in 1979. Eric Clapton also recorded Just One Night at the venue, released in 1980. ABBA held their final ever live performance at the Budokan in March 1980.
The Nippon Budokan is modeled after Yumedono, the Hall of Dreams, at the temple complex of Horyuji in Nara. It takes an octagonal form and stands 42 meters tall in Kitanomaru Park, two minutes on foot from Kudanshita Subway Station.