The grounds of Usa Hachimangu are like a large open park, but it wasnt always that way. Until 1868 the grounds were packed with dozens and dozens of structures,... in a scale model I counted at least eight pagodas...., for this was a huge shrine-temple complex until the government "seperated" the Buddhas and Kami and the buildings were either torn down or dismantled.
The Tongu is a newer structure built where part of the earlier temple complex stood. The Tongu is unused for 362 days of the year, only being used from July 31st to August 2nd when it is home to 3 mikoshi as part of the Nagoshi Festival.
The mikoshi are carried here from the main shrine buildings during which time the three mikoshi battle each other to see who will lead the procession. Once at the Tongu rituals are held. On the third day the mikoshi are taken back up the hill to the main shrine.