This is not a ruin, but still a pretty interesting historical Civil War location in Williamsport, Maryland. In April 1861, Williamsport residents awoke to find themselves living across the Potomac River from the enemy. Virginia had seceded from the Union and troops from both sides skirmished for control over the vital river crossing and the C&O Canal. The town had to build fortifications to protect the Union resources. Fort Sumter hero, Major Abner Doubleday arrived in June 1861 and ordered the placement of three siege guns, and the construction of an ammunition storage bunker and other fortifications on the hill overlooking the town and river. Even though Doubleday was transferred to Harpers Ferry a few weeks later, his name is still associated with the hill.
Abner Doubleday is credited with inventing the game of baseball in 1835.