The Library of Congress has this amazing panorama of Detroit's Campus Martius taken in 1914. You can click the photo above for a large panorama and get details on the photograph here (and a link to a 12 MB tiff of the panorama)!!
From the Campus Martius Park web site:
In 1788, Campus Martius served as a drill ground for militia training. Campus Martius means "military ground" and was named after the Campus Martius at Marietta, Ohio, a 180-foot stockade.
After the fire of 1805, Judge Woodward was appointed to oversee the plan to lay out the streets, squares and lots with the assistance of the best surveyors from Canada. They placed their instruments and astronomical devices on the summit of a huge stone. He viewed the planets and meteors in order to determine "true North". Today we still call this point the "Point of Origin" which is located in center of Campus Martius at the junction of Woodward and Monroe. It is from this point that the City of Detroit's Coordinate system was created.
UrbanTiki explains that when it's not hosting the Motown Winter Blast, Campus Martius's hydraulic concert stage allows it to host concerts (photo by ifmuth) and serve as an ice rink (photo by mainegal). SNWEB.org has a great shot of the Point of Origin. (sorry to get all link-happy on you there)
Speaking of link-happy, here's a look by Michigan in Pictures at the Michigan Soldiers & Sailors Monument which is located in Campus Martius.