Part of the old federal fort built on the remains of a fort established by the British in 1779
The arsenal housed the powder for guns used in the defense of the US southern border.

The building was built in 1838 and is listed on the National Register of Historic Places

For more information see http://www.sos.la.gov/HistoricalResources/VisitMuseums/OldArsenalMuseum/Pages/default.aspx

also
http://www.exploresouthernhistory.com/powdermagazine.htmlalso