When a small Confederate army set up camp on the site of what is now Saint Louis University, their plot to seize the weapons stored at the Arsenal south of downtown began to unfold. However, they were stopped by Nathaniel Lyon, who commanded a Union Army that swept in and captured the Confederates, protecting the Arsenal for the Union. The ensuing withdrawal back to the Arsenal with the prisoners of war resulted in the deaths of dozens of Confederate sympathizers who attempted to block the column in the streets of the city.
The Arsenal has always fascinated me since I discovered it a few years back; why I did not assume St. Louis had one since there is a street with the same name is beyond me.
While it is now the Defense Mapping Agency for the US military, it played a critical role in the fight for the control of St. Louis during the Civil War, which was always in question throughout the war.
Lyon Park, named after Nathaniel Lyon for his role in saving the Arsenal for the Union, was already platted out several decades after the Civil War. Lyon was later killed at the Battle of Wilson’s Creek.